Code Interview

Linear time algorithm for finding longest palindrome substring

2012-05-28T22:13:00.000-07:00

Linear time algorithm for finding

longest palindrome substring

Code in C++

char str[MAXSIZE];//pre-processed array

int p[MAXSIZE]; //auxiliary array

char s[MAXSIZE];//original input

int n;

void populatep()

{

int i;

int mx = 0;

int id;

for(i=1; i<n; i++)

{

if( mx > i )

p[i] = MIN( p[2*id-i], p[id]+id-i );

else

p[i] = 1;

for(; str[i+p[i]] == str[i-p[i]]; p[i]++)

;

if( p[i] + i > mx )

{

mx = p[i] + i;

id = i;

}

void preprocess()

{

int i,j,k;

n = strlen(s);

str[0] = '$';

str[1] = '#';

for(i=0;i<n;i++)

{

str[i*2 + 2] = s[i];

str[i*2 + 3] = '#';

}

n = n*2 + 2;

str[n] = 0;

cout <<str<<" : "<<strlen(str)<<endl;

}

void getlength()

{

int i;

int ans = 0;

for(i=0;i<n;i++)

getmax(ans, p[i]);

printf("%d\n", ans-1);

}

//test

int main()

{

while( canf("%s", s) )

{

preprocess();

populatep();

getlength();

}

return 0;

}

For background, please read Wikipedia’s article about Longest Palindromic Substring.

Relevant links: A simple linear time algorithm for finding longest palindrome sub-string

http://www.leetcode.com/2011/11/longest-palindromic-substring-part-ii.html#comment-20368

http://zhuhongcheng.wordpress.com/2009/08/02/a-simple-linear-time-algorithm-for-finding-longest-palindrome-sub-string/

Python implementation: http://www.akalin.cx/longest-palindrome-linear-time

A better emplanation in Chinese : http://blog.csdn.net/ggggiqnypgjg/article/details/6645824

How To Read C Declarations

2012-05-28T21:41:00.001-07:00

Even experienced C programmers have difficulty reading declarations that go beyond simple arrays and pointers. For example, is the following an array of pointers or a pointer to an array?

int *a[10];

What the heck does the following mean?

int (*(*vtable)[])();

Naturally, it's a pointer to an array of pointers to functions returning integers.

This short article tells you how to read any C declaration correctly using a very simple technique. I am 99% certain that I read this in a book in the late 1980s, but I can't remember where. I doubt that I discovered this on my own (even though I've always been delighted by computer language structure and esoterica). I do remember, however, building a simple program that would translate any declaration into English.

The golden rule

The rule goes like this:

Start at the variable name (or innermost construct if no identifier
is present. Look right without jumping over a right parenthesis; say
what you see. Look left again without jumping over a parenthesis; say
what you see. Jump out a level of parentheses if any. Look right;
say what you see. Look left; say what you see. Continue in this
manner until you say the variable type or return type.

The degenerate case is:

int i;

Starting at i , you look right and find nothing. You look left and find the type int , which you say. Done.

Ok, now a more complicated one:

int *a[3];

Start at a . Look right, say array of size 3. Look left and say pointer. Look right and see nothing. Look left and say int. All together you say a is an array of size 3 pointers to int.

Adding parentheses is when it gets weird:

int (*a)[3];

The parentheses change the order just like in an expression. When you look right after a , you see the right parenthesis, which you cannot jump over until you look left. Hence, you would say a is a pointer to an array of 3 ints.

Function pointers

The C "forward" declaration:

extern int foo();

just says that foo is a function returning int. This follows the same pattern for reading declarators as you saw in previous section. Start at foo and look right. You see () so say function. You look left and see int . Say int.

Now, try this one:

extern int *foo();

Yep, you say foo is a function returning a pointer to int.

Now for the big leap. Just like we can make a pointer to an int or whatever, let's make a pointer to a function. In this case, we can drop the extern as it's no longer a function forward reference, but a data variable declaration. Here is the basic pattern for function pointer:

int (*foo)();

You start at foo and see nothing to the right. So, to the left, you say pointer. Then to the right outside you see function. Then left you see int . So you say foo is a pointer to a function returning int.

Combinations

Here is an array of pointers to functions returning int, which we'll need for vtables below:

int (*Object_vtable[])();

You need one last, incredibly bizarre declaration, for the lab:

int (*(*vtable)[])();

This is the pointer to the vtable you will need in each "object" you define.

This pointer to a vtable is set to the address of a vtable; for example, &Truck_vtable .

Summary

The following examples summarize the cases needed for building virtual tables ala C++ to implement polymorphism (like the original cfront C++ to C translator).

int *ptr_to_int;

int *func_returning_ptr_to_int();

int (*ptr_to_func_returning_int)();

int (*array_of_ptr_to_func_returning_int[])();

int (*(*ptr_to_an_array_of_ptr_to_func_returning_int)[])();

Source: click here Author Terence Parr

The great advice from John Bode

It's easier to remember the rule as absent any explicit grouping, () and [] bind before *. Thus, for a declaration like

T *a[N];

the [] bind before the *, so a is an N-element array of pointer. Breaking it down in steps:

   a     -- a
   a[N]  -- is an N-element array
  *a[N]  -- of pointer
T *a[N]  -- to T.

For a declaration like

T (*a)[N];

the parens force the * to bind before the [], so

    a      -- a
  (*a)     -- is a pointer
  (*a)[N]  -- to an N-element array
T (*a)[N]  -- of T

It's still the clockwise/spiral rule, just expressed in a more compact manner.

Java Interview Frequently Asked Questions & Answers (S1P2

2012-04-27T17:37:00.000-07:00

Difference between Vector and ArrayList?

Vector is synchronized whereas arraylist is not.

Difference between Swing and Awt?

AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT.

What is the difference between a constructor and a method?

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

What is an Iterator?

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

public : Public class is visible in other packages, field is visible everywhere (class must be public too)

private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.

protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.

default :What you get by default, i.e., without any access modifier (i.e., public private or protected).It means that it is visible to all within a particular package.

What is an abstract class?

Abstract class must be extended/sub-classed (to be useful). It serves as a template. A class that is abstract may not be instantiated (i.e., you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.

A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

What is static in java?

Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

What is final?

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Source: Here

Java Interview Frequently Asked Questions & Answers (S1P1)

2012-04-27T17:32:00.000-07:00

What is the difference between an Interface and an Abstract class?

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

What is the purpose of garbage collection in Java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Describe synchronization in respect to multithreading.

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Explain different way of using thread?

The thread could be implemented by using runable interface or by inheriting from the Thread class. The former is more advantageous, because when you are going for multiple inheritance, the only interface can help.

What are pass by reference and pass by value?

Pass By Reference means the passing the address itself rather than passing the value. Pass by Value means passing a copy of the value to be passed.

What is HashMap and Map?

Map is Interface and Hashmap is class that implements that.

Difference between HashMap and HashTable?

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable does not allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

Source: Here

Java Interview Frequently Asked Questions & Answers (Part Six)

2012-04-26T18:21:00.001-07:00

What is RMI architecture?- RMI architecture consists of four layers and each layer performs specific functions: a) Application layer - contains the actual object definition. b) Proxy layer - consists of stub and skeleton. c) Remote Reference layer - gets the stream of bytes from the transport layer and sends it to the proxy layer. d) Transportation layer - responsible for handling the actual machine-to-machine communication.
what is UnicastRemoteObject?- All remote objects must extend UnicastRemoteObject, which provides functionality that is needed to make objects available from remote machines.
Explain the methods, rebind() and lookup() in Naming class?- rebind() of the Naming class(found in java. rmi) is used to update the RMI registry on the server machine. Naming. rebind(”AddSever”, AddServerImpl); lookup() of the Naming class accepts one argument, the rmi URL and returns a reference to an object of type AddServerImpl.
What is a Java Bean?- A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
What is a Jar file?- Jar file allows to efficiently deploying a set of classes and their associated resources. The elements in a jar file are compressed, which makes downloading a Jar file much faster than separately downloading several uncompressed files. The package java. util. zip contains classes that read and write jar files.
What is BDK?- BDK, Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and it can be used to test Beans without writing a code.
What is JSP?- JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can’t do any client side validation with it. The advantages are: a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process. b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.
What are JSP scripting elements?- JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page. There are three forms: a) Expressions of the form <%= expression %> that are evaluated and inserted into the output, b) Scriptlets of the form<% code %>that are inserted into the servlet’s service method, and c) Declarations of the form <%! Code %>that are inserted into the body of the servlet class, outside of any existing methods.
What are JSP Directives?- A JSP directive affects the overall structure of the servlet class. It usually has the following form:<%@ directive attribute=”value” %> However, you can also combine multiple attribute settings for a single directive, as follows:<%@ directive attribute1=”value1″ attribute 2=”value2″ . . . attributeN =”valueN” %> There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet
What are Predefined variables or implicit objects?- To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes called implicit objects. They are request, response, out, session, application, config, pageContext, and page.
What are JSP ACTIONS?- JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: jsp:include - Include a file at the time the page is requested. jsp:useBean - Find or instantiate a JavaBean. jsp:setProperty - Set the property of a JavaBean. jsp:getProperty - Insert the property of a JavaBean into the output. jsp:forward - Forward the requester to a newpage. Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED
How do you pass data (including JavaBeans) to a JSP from a servlet?- (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either “include” or forward”) can be called. This bean will disappear after processing this request has been completed. Servlet: request. setAttribute(”theBean”, myBean); RequestDispatcher rd = getServletContext(). getRequestDispatcher(”thepage. jsp”); rd. forward(request, response); JSP PAGE:<jsp: useBean id=”theBean” scope=”request” class=”. . . . . ” />(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it. Servlet: HttpSession session = request. getSession(true); session. putValue(”theBean”, myBean); /* You can do a request dispatcher here, or just let the bean be visible on the next request */ JSP Page:<jsp:useBean id=”theBean” scope=”session” class=”. . . ” /> 3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it. Servlet: GetServletContext(). setAttribute(”theBean”, myBean); JSP PAGE:<jsp:useBean id=”theBean” scope=”application” class=”. . . ” />
How can I set a cookie in JSP?- response. setHeader(”Set-Cookie”, “cookie string”); To give the response-object to a bean, write a method setResponse (HttpServletResponse response) - to the bean, and in jsp-file:<% bean. setResponse (response); %>
How can I delete a cookie with JSP?- Say that I have a cookie called “foo, ” that I set a while ago & I want it to go away. I simply: <% Cookie killCookie = new Cookie(”foo”, null); KillCookie. setPath(”/”); killCookie. setMaxAge(0); response. addCookie(killCookie); %>
How are Servlets and JSP Pages related?- JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed.

Source:Click here

Java Interview Frequently Asked Questions & Answers (Part Five)

2012-04-26T18:20:00.000-07:00

What is the difference between an applet and a servlet?- a) Servlets are to servers what applets are to browsers. b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
What is the difference between doPost and doGet methods?- a) doGet() method is used to get information, while doPost() method is used for posting information. b) doGet() requests can’t send large amount of information and is limited to 240-255 characters. However, doPost()requests passes all of its data, of unlimited length. c) A doGet() request is appended to the request URL in a query string and this allows the exchange is visible to the client, whereas a doPost() request passes directly over the socket connection as part of its HTTP request body and the exchange are invisible to the client.
What is the life cycle of a servlet?- Each Servlet has the same life cycle: a) A server loads and initializes the servlet by init () method. b) The servlet handles zero or more client’s requests through service() method. c) The server removes the servlet through destroy() method.
Who is loading the init() method of servlet?- Web server
What are the different servers available for developing and deploying Servlets?- a) Java Web Server b) JRun g) Apache Server h) Netscape Information Server i) Web Logic
How many ways can we track client and what are they?- The servlet API provides two ways to track client state and they are: a) Using Session tracking and b) Using Cookies.
What is session tracking and how do you track a user session in servlets?- Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are: a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password. b) Hidden form fields - fields are added to an HTML form that are not displayed in the client’s browser. When the form containing the fields is submitted, the fields are sent back to the server. c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change. d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser. e) HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session. maxresidents property.
What is Server-Side Includes (SSI)?- Server-Side Includes allows embedding servlets within HTML pages using a special servlet tag. In many servlets that support servlets, a page can be processed by the server to include output from servlets at certain points inside the HTML page. This is accomplished using a special internal SSINCLUDE, which processes the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an. shtml extension is requested. So HTML files that include server-side includes must be stored with an . shtml extension.
What are cookies and how will you use them?- Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information associated with the user. a) Create a cookie with the Cookie constructor: public Cookie(String name, String value) b) A servlet can send a cookie to the client by passing a Cookie object to the addCookie() method of HttpServletResponse: public void HttpServletResponse. addCookie(Cookie cookie) c) A servlet retrieves cookies by calling the getCookies() method of HttpServletRequest: public Cookie[ ] HttpServletRequest. getCookie().
Is it possible to communicate from an applet to servlet and how many ways and how?- Yes, there are three ways to communicate from an applet to servlet and they are: a) HTTP Communication(Text-based and object-based) b) Socket Communication c) RMI Communication
What is connection pooling?- With servlets, opening a database connection is a major bottleneck because we are creating and tearing down a new connection for every page request and the time taken to create connection will be more. Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also intelligently manage the size of the pool and make sure each connection remains valid. A number of connection pool packages are currently available. Some like DbConnectionBroker are freely available from Java Exchange Works by creating an object that dispenses connections and connection Ids on request. The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as stored values. The Boolean value indicates whether a connection is in use or not. A program calls getConnection() method of the ConnectionPool for getting Connection object it can use; it calls returnConnection() to give the connection back to the pool.
Why should we go for interservlet communication?- Servlets running together in the same server communicate with each other in several ways. The three major reasons to use interservlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation)
Is it possible to call servlet with parameters in the URL?- Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy).
What is Servlet chaining?- Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet’s output is piped to the next servlet’s input. This process continues until the last servlet is reached. Its output is then sent back to the client.
How do servlets handle multiple simultaneous requests?- The server has multiple threads that are available to handle requests. When a request comes in, it is assigned to a thread, which calls a service method (for example: doGet(), doPost() and service()) of the servlet. For this reason, a single servlet object can have its service methods called by many threads at once.
What is the difference between TCP/IP and UDP?- TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. It is like a phone call. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination. It is like a postal mail.
What is Inet address?- Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
What is Domain Naming Service(DNS)?- It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters. For example, www. mascom. com implies com is the domain name reserved for US commercial sites, moscom is the name of the company and www is the name of the specific computer, which is mascom’s server.
What is URL?- URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www. address. com:80/index.html, where http - protocol name, address - IP address or host name, 80 - port number and index.html - file path.
What is RMI and steps involved in developing an RMI object?- Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application.

Source:Click here

Java Interview Frequently Asked Questions & Answers (Part Four)

2012-04-26T18:18:00.004-07:00

Which containers use a Border layout as their default layout?- Window, Frame and Dialog classes use a BorderLayout as their layout.
Which containers use a Flow layout as their default layout?- Panel and Applet classes use the FlowLayout as their default layout.
What are wrapper classes?- Wrapper classes are classes that allow primitive types to be accessed as objects.
What are Vector, Hashtable, LinkedList and Enumeration?- Vector : The Vector class provides the capability to implement a growable array of objects. Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object’s keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations. Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements() and nextElement(). HasMoreElemnts() tests if this enumeration has more elements and nextElement method returns successive elements of the series.
What is the difference between set and list?- Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.
What is a stream and what are the types of Streams and classes of the Streams?- A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.
What is the difference between Reader/Writer and InputStream/Output Stream?- The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
What is an I/O filter?- An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
What is serialization and deserialization?- Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
What is JDBC?- JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.
What are drivers available?- a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver
What is the difference between JDBC and ODBC?- a) OBDC is for Microsoft and JDBC is for Java applications. b) ODBC can’t be directly used with Java because it uses a C interface. c) ODBC makes use of pointers which have been removed totally from Java. d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required. e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.
What are the types of JDBC Driver Models and explain them?- There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above.
What are the steps involved for making a connection with a database or how do you connect to a database?a) Loading the driver : To load the driver, Class. forName() method is used. Class. forName(”sun. jdbc. odbc. JdbcOdbcDriver”); When the driver is loaded, it registers itself with the java. sql. DriverManager class as an available database driver. b) Making a connection with database: To open a connection to a given database, DriverManager. getConnection() method is used. Connection con = DriverManager. getConnection (”jdbc:odbc:somedb”, “user”, “password”); c) Executing SQL statements : To execute a SQL query, java. sql. statements class is used. createStatement() method of Connection to obtain a new Statement object. Statement stmt = con. createStatement(); A query that returns data can be executed using the executeQuery() method of Statement. This method executes the statement and returns a java. sql. ResultSet that encapsulates the retrieved data: ResultSet rs = stmt. executeQuery(”SELECT * FROM some table”); d) Process the results : ResultSet returns one row at a time. Next() method of ResultSet object can be called to move to the next row. The getString() and getObject() methods are used for retrieving column values: while(rs. next()) { String event = rs. getString(”event”); Object count = (Integer) rs. getObject(”count”);
What type of driver did you use in project?- JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).
What are the types of statements in JDBC?- Statement: to be used createStatement() method for executing single SQL statement PreparedStatement — To be used preparedStatement() method for executing same SQL statement over and over. CallableStatement — To be used prepareCall() method for multiple SQL statements over and over.
What is stored procedure?- Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.
How to create and call stored procedures?- To create stored procedures: Create procedure procedurename (specify in, out and in out parameters) BEGIN Any multiple SQL statement; END; To call stored procedures: CallableStatement csmt = con. prepareCall(”{call procedure name(?,?)}”); csmt. registerOutParameter(column no. , data type); csmt. setInt(column no. , column name) csmt. execute();
What is servlet?- Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company’s order database.

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Java Interview Frequently Asked Questions & Answers (Part Three)

2012-04-26T18:13:00.003-07:00

What is synchronization?- Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.
When you will synchronize a piece of your code?- When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.
What is deadlock?- When two threads are waiting each other and can’t precede the program is said to be deadlock.
What is daemon thread and which method is used to create the daemon thread?- Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
Are there any global variables in Java, which can be accessed by other part of your program?- No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.
What is an applet?- Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.
What is the difference between applications and applets?- a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
How does applet recognize the height and width?- Using getParameters() method.
When do you use codebase in applet?- When the applet class file is not in the same directory, codebase is used.
What is the lifecycle of an applet?- init() method - Can be called when an applet is first loaded start() method - Can be called each time an applet is started. paint() method - Can be called when the applet is minimized or maximized. stop() method - Can be used when the browser moves off the applet’s page. destroy() method - Can be called when the browser is finished with the applet.
How do you set security in applets?- using setSecurityManager() method
What is an event and what are the models available for event handling?- An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model
What are the advantages of the model over the event-inheritance model?- The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.
What is source and listener?- source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.
What is adapter class?- An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged()and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged() .
What is meant by controls and what are different types of controls in AWT?- Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.
What is the difference between choice and list?- A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.
What is the difference between scrollbar and scrollpane?- A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.
What is a layout manager and what are different types of layout managers available in java AWT?- A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
How are the elements of different layouts organized?- FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

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Java Interview Frequently Asked Questions & Answers (Part Two)

2012-04-25T16:35:00.001-07:00

What is meant by Inheritance and what are its advantages?- Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
What is the difference between this() and super()?- this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
What is the difference between superclass and subclass?- A super class is a class that is inherited whereas sub class is a class that does the inheriting.
What modifiers may be used with top-level class?- public, abstract and final can be used for top-level class.
What are inner class and anonymous class?- Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private. Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
What is a package?- A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
What is a reflection package?- java. lang. reflect package has the ability to analyze itself in runtime.
What is interface and its use?- Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object’s programming interface without revealing the actual body of the class.
What is an abstract class?- An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
What is the difference between Integer and int?- a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
What is a cloneable interface and how many methods does it contain?- It is not having any method because it is a TAGGED or MARKER interface.
What is the difference between abstract class and interface?- a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can’t have subclasses.
Can you have an inner class inside a method and what variables can you access?- Yes, we can have an inner class inside a method and final variables can be accessed.
What is the difference between String and String Buffer?- a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
What is the difference between Array and vector?- Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
What is the difference between exception and error?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing. The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.
What is the difference between process and thread?- Process is a program in execution whereas thread is a separate path of execution in a program.
What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?- Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state. notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.
What is the class and interface in java to create thread and which is the most advantageous method?- Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
What are the states associated in the thread?- Thread contains ready, running, waiting and dead states.

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Java Interview Frequently Asked Questions & Answers (Part One)

2012-04-25T16:30:00.004-07:00

What is the difference between procedural and object-oriented programs?- a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP program, unit of program is object, which is nothing but combination of data and code. b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.
What are Encapsulation, Inheritance and Polymorphism?- Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.
What is the difference between Assignment and Initialization?- Assignment can be done as many times as desired whereas initialization can be done only once.
What is OOPs?- Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.
What are Class, Constructor and Primitive data types?- Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.
What is an Object and how do you allocate memory to it?- Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
What is the difference between constructor and method?- Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
What are methods and how are they defined?- Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method’s signature is a combination of the first three parts mentioned above.
What is the use of bin and lib in JDK?- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
What is casting?- Casting is used to convert the value of one type to another.
How many ways can an argument be passed to a subroutine and explain them?- An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
What is the difference between an argument and a parameter?- While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
What are different types of access modifiers?- public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can’t be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.
What is final, finalize() and finally?- final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value. finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection. finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.
What is UNICODE?- Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
What is Garbage Collection and how to call it explicitly?- When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System. gc() method may be used to call it explicitly.
What is finalize() method?- finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.
What are Transient and Volatile Modifiers?- Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
What is method overloading and method overriding?- Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
What is difference between overloading and overriding?- a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method. b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass. c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass. d) Overloading must have different method signatures whereas overriding must have same signature.

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C++ frequently asked questions (4)

2012-03-24T18:04:00.000-07:00

What is multiple inheritance(virtual inheritance)? What are its advantages and disadvantages?

Multiple Inheritance is the process whereby a child can be derived from more than one parent class. The advantage of multiple inheritance is that it allows a class to inherit the functionality of more than one base class thus allowing for modeling of complex relationships. The disadvantage of multiple inheritance is that it can lead to a lot of confusion(ambiguity) when two base classes implement a method with the same name.

What are the differences between a C++ struct and C++ class?

The default member and base class access specifiers are different.

The C++ struct has all the features of the class. The only differences are that a struct defaults to public member access and public base class inheritance, and a class defaults to the private access specifier and private base class inheritance.

What are the access privileges in C++? What is the default access level?

The access privileges in C++ are private, public and protected. The default access level assigned to members of a class is private. Private members of a class are accessible only within the class and by friends of the class. Protected members are accessible by the class itself and it's sub-classes. Public members of a class can be accessed by anyone.

What is a nested class? Why can it be useful?

A nested class is a class enclosed within the scope of another class. For example:

// Example 1: Nested class
//
class OuterClass
{
class NestedClass
{
// ...
};
// ...
};
Nested classes are useful for organizing code and controlling access and dependencies. Nested classes obey access rules just like other parts of a class do; so, in Example 1, if NestedClass is public then any code can name it as OuterClass::NestedClass. Often nested classes contain private implementation details, and are therefore made private; in Example 1, if NestedClass is private, then only OuterClass's members and friends can use NestedClass.

When you instantiate as outer class, it won't instantiate inside class.

What is a local class? Why can it be useful?

local class is a class defined within the scope of a function -- any function, whether a member function or a free function. For example:

// Example 2: Local class
//
int f()
{
class LocalClass
{
// ...
};
// ...
};
Like nested classes, local classes can be a useful tool for managing code dependencies.

Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

No. It is specified in the definition of the copy constructor itself. It should generate an error if a programmer specifies a copy constructor with a first argument that is an object and not a reference.

C++ frequently asked questions (3) - Pointer and Memory management

2012-03-24T18:02:00.000-07:00

What is Memory alignment??

The term alignment primarily means the tendency of an address pointer value to be a multiple of some power of two. So a pointer with two byte alignment has a zero in the least significant bit. And a pointer with four byte alignment has a zero in both the two least significant bits. And so on. More alignment means a longer sequence of zero bits in the lowest bits of a pointer.

What problem does the namespace feature solve?

Multiple providers of libraries might use common global identifiers causing a name collision when an application tries to link with two or more such libraries. The namespace feature surrounds a library's external declarations with a unique namespace that eliminates the potential for those collisions.

namespace [identifier] { namespace-body }

A namespace declaration identifies and assigns a name to a declarative region.
The identifier in a namespace declaration must be unique in the declarative region in which it is used. The identifier is the name of the namespace and is used to reference its members.

What is an Iterator class?

A class that is used to traverse through the objects maintained by a container class. There are five categories of iterators: input iterators, output iterators, forward iterators, bidirectional iterators, random access. An iterator is an entity that gives access to the contents of a container object without violating encapsulation constraints. Access to the contents is granted on a one-at-a-time basis in order. The order can be storage order (as in lists and queues) or some arbitrary order (as in array indices) or according to some ordering relation (as in an ordered binary tree). The iterator is a construct, which provides an interface that, when called, yields either the next element in the container, or some value denoting the fact that there are no more elements to examine. Iterators hide the details of access to and update of the elements of a container class. Something like a pointer.

What is the use of 'using' declaration?

A using declaration makes it possible to use a name from a namespace without the scope operator.

What is a dangling pointer?

A dangling pointer arises when you use the address of an object after its lifetime is over. This may occur in situations like returning addresses of the automatic variables from a function or using the address of the memory block after it is freed.

What do you mean by Stack unwinding?

It is a process during exception handling when the destructor is called for all local objects in the stack between the place where the exception was thrown and where it is caught.

Name the operators that cannot be overloaded??

sizeof, ., .*, .->, ::, ?:

What is overloading??

With the C++ language, you can overload functions and operators. Overloading is the practice of supplying more than one definition for a given function name in the same scope.

- Any two functions in a set of overloaded functions must have different argument lists.
- Overloading functions with argument lists of the same types, based on return type alone, is an error.

What is inline function??

The __inline keyword tells the compiler to substitute the code within the function definition for every instance of a function call. However, substitution occurs only at the compiler's discretion. For example, the compiler does not inline a function if its address is taken or if it is too large to inline.

What is a container class? What are the types of container classes?

A container class is a class that is used to hold objects in memory or external storage. A container class acts as a generic holder. A container class has a predefined behavior and a well-known interface. A container class is a supporting class whose purpose is to hide the topology used for maintaining the list of objects in memory. When a container class contains a group of mixed objects, the container is called a heterogeneous container; when the container is holding a group of objects that are all the same, the container is called a homogeneous container.

What is Overriding?

To override a method, a subclass of the class that originally declared the method must declare a method with the same name, return type (or a subclass of that return type), and same parameter list.
The definition of the method overriding is:
· Must have same method name.
· Must have same data type.
· Must have same argument list.
Overriding a method means that replacing a method functionality in child class. To imply overriding functionality we need parent and child classes. In the child class you define the same method signature as one defined in the parent class.

What is "this" pointer?

The this pointer is a pointer accessible only within the member functions of a class, struct, or union type. It points to the object for which the member function is called. Static member functions do not have a this pointer.

When a nonstatic member function is called for an object, the address of the object is passed as a hidden argument to the function. For example, the following function call

myDate.setMonth( 3 );

can be interpreted this way:

setMonth( &myDate, 3 );

The object's address is available from within the member function as the this pointer. It is legal, though unnecessary, to use the this pointer when referring to members of the class.

How virtual functions are implemented C++?

Virtual functions are implemented using a table of function pointers, called the vtable. There is one entry in the table per virtual function in the class. This table is created by the constructor of the class. When a derived class is constructed, its base class is constructed first which creates the vtable. If the derived class overrides any of the base classes virtual functions, those entries in the vtable are overwritten by the derived class constructor. This is why you should never call virtual functions from a constructor: because the vtable entries for the object may not have been set up by the derived class constructor yet, so you might end up calling base class implementations of those virtual functions

What is name mangling in C++??

The process of encoding the parameter types with the function/method name into a unique name is called name mangling. The inverse process is called demangling.

For example Foo::bar(int, long) const is mangled as `bar__C3Fooil'.
For a constructor, the method name is left out. That is Foo::Foo(int, long) const is mangled as `__C3Fooil'.

What happens when you make call "delete this;" ??

The code has two built-in pitfalls. First, if it executes in a member function for an extern, static, or automatic object, the program will probably crash as soon as the delete statement executes. There is no portable way for an object to tell that it was instantiated on the heap, so the class cannot assert that its object is properly instantiated. Second, when an object commits suicide this way, the using program might not know about its demise. As far as the instantiating program is concerned, the object remains in scope and continues to exist even though the object did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster.

You should never do this. Since compiler does not know whether the object was allocated on the stack or on the heap, "delete this" could cause a disaster.

What is the difference between a pointer and a reference?

A reference must always refer to some object and, therefore, must always be initialized; pointers do not have such restrictions. A pointer can be reassigned to point to different objects while a reference always refers to an object with which it was initialized.

How are prefix and postfix versions of operator++() differentiated?

The postfix version of operator++() has a dummy parameter of type int. The prefix version does not have dummy parameter.

What is the difference between const char *myPointer and char *const myPointer?

Const char *myPointer is a non constant pointer to constant data; while char *const myPointer is a constant pointer to non constant data.

How can I handle a constructor that fails?

throw an exception. Constructors don't have a return type, so it's not possible to use return codes. The best way to signal constructor failure is therefore to throw an exception.

How can I handle a destructor that fails?

Write a message to a log-file. But do not throw an exception.
The C++ rule is that you must never throw an exception from a destructor that is being called during the "stack unwinding" process of another exception. For example, if someone says throw Foo(), the stack will be unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get popped. This is called stack unwinding.
During stack unwinding, all the local objects in all those stack frames are destructed. If one of those destructors throws an exception (say it throws a Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a } catch (Bar e) { handler? There is no good answer -- either choice loses information.
So the C++ language guarantees that it will call terminate() at this point, and terminate() kills the process. Bang you're dead.

What is Virtual Destructor?

Using virtual destructors, you can destroy objects without knowing their type - the correct destructor for the object is invoked using the virtual function mechanism. Note that destructors can also be declared as pure virtual functions for abstract classes.

if someone will derive from your class, and if someone will say "new Derived", where "Derived" is derived from your class, and if someone will say delete p, where the actual object's type is "Derived" but the pointer p's type is your class.

Can you think of a situation where your program would crash without reaching the breakpoint which you set at the beginning of main()?

C++ allows for dynamic initialization of global variables before main() is invoked. It is possible that initialization of global will invoke some function. If this function crashes the crash will occur before main() is entered.

Name two cases where you MUST use initialization list as opposed to assignment in constructors.

Both non-static const data members and reference data members cannot be assigned values; instead, you should use initialization list to initialize them.

Can you overload a function based only on whether a parameter is a value or a reference?

No. Passing by value and by reference looks identical to the caller.

What does extern "C" int func(int *, Foo) accomplish?

It will turn off "name mangling" for func so that one can link to code compiled by a C compiler.

How do you access the static member of a class?

C++ frequently asked questions (2)

2012-03-24T17:59:00.000-07:00

What is the diff between malloc()/free() and new/delete?

malloc allocates memory for object in heap but doesn't invoke object's constructor to initiallize the object.

new allocates memory and also invokes constructor to initialize the object.

malloc() and free() do not support object semantics
Does not construct and destruct objects
string * ptr = (string *)(malloc (sizeof(string)))
Are not safe
Does not calculate the size of the objects that it construct
Returns a pointer to void
int *p = (int *) (malloc(sizeof(int)));
int *p = new int;
Are not extensible
new and delete can be overloaded in a class

"delete" first calls the object's termination routine (i.e. its destructor) and then releases the space the object occupied on the heap memory. If an array of objects was created using new, then delete must be told that it is dealing with an array by preceding the name with an empty []:-

Int_t *my_ints = new Int_t[10];

...

delete []my_ints;

what is the diff between "new" and "operator new" ?

"operator new" works like malloc.

What is difference between template and macro??

There is no way for the compiler to verify that the macro parameters are of compatible types. The macro is expanded without any special type checking.

If macro parameter has a postincremented variable ( like c++ ), the increment is performed two times.

Because macros are expanded by the preprocessor, compiler error messages will refer to the expanded macro, rather than the macro definition itself. Also, the macro will show up in expanded form during debugging.

for example:

Macro:

#define min(i, j) (i <>

template:
template
T min (T i, T j)
{
return i <>

What are storage qualifiers in C++ ?

They are..

const
volatile
mutable

Const keyword indicates that memory once initialized, should not be altered by a program.

volatile keyword indicates that the value in the memory location can be altered even though nothing in the program
code modifies the contents. for example if you have a pointer to hardware location that contains the time, where hardware changes the value of this pointer variable and not the program. The intent of this keyword to improve the optimization ability of the compiler.

mutable keyword indicates that particular member of a structure or class can be altered even if a particular structure variable, class, or class member function is constant.

struct data
{
char name[80];
mutable double salary;
}

const data MyStruct = { "Satish Shetty", 1000 }; //initlized by complier

strcpy ( MyStruct.name, "Shilpa Shetty"); // compiler error
MyStruct.salaray = 2000 ; // complier is happy allowed

What is reference ??

reference is a name that acts as an alias, or alternative name, for a previously defined variable or an object.

prepending variable with "&" symbol makes it as reference.

for example:

int a;
int &b = a;

What are C++ storage classes?

auto
register
static
extern

auto: the default. Variables are automatically created and initialized when they are defined and are destroyed at the end of the block containing their definition. They are not visible outside that block

register: a type of auto variable. a suggestion to the compiler to use a CPU register for performance

static: a variable that is known only in the function that contains its definition but is never destroyed and retains its value between calls to that function. It exists from the time the program begins execution

extern: a static variable whose definition and placement is determined when all object and library modules are combined (linked) to form the executable code file. It can be visible outside the file where it is defined.

What is passing by reference?

Method of passing arguments to a function which takes parameter of type reference.

for example:

void swap( int & x, int &amp;amp;amp; y )
{
int temp = x;
x = y;
y = temp;
}

int a=2, b=3;

swap( a, b );

Basically, inside the function there won't be any copy of the arguments "x" and "y" instead they refer to original variables a and b. so no extra memory needed to pass arguments and it is more efficient.

When do use "const" reference arguments in function?

a) Using const protects you against programming errors that inadvertently alter data.
b) Using const allows function to process both const and non-const actual arguments, while a function without const in the prototype can only accept non constant arguments.
c) Using a const reference allows the function to generate and use a temporary variable appropriately.

What is virtual function?

When derived class overrides the base class method by redefining the same function, then if client wants to access redefined the method from derived class through a pointer from base class object, then you must define this function in base class as virtual function.

class parent
{
void Show()
{
cout << "i'm parent" << endl;
}
};

class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}

};

parent * parent_object_ptr = new child;

parent_object_ptr->show() // calls parent->show() i

now we goto virtual world...

class parent
{
virtual void Show()
{
cout << "i'm parent" << endl;
}
};

class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}

};

parent * parent_object_ptr = new child;

parent_object_ptr->show() // calls child->show()

When are temporary variables created by C++ compiler?

Provided that function parameter is a "const reference", compiler generates temporary variable in following 2 ways.

a) The actual argument is the correct type, but it isn't Lvalue

double Cube(const double & num)
{
num = num * num * num;
return num;

}

double temp = 2.0;
double value = cube(3.0 + temp); // argument is a expression and not a Lvalue;

b) The actual argument is of the wrong type, but of a type that can be converted to the correct type

long temp = 3L;
double value = cuberoot ( temp); // long to double conversion

What is pure virtual function? or what is abstract class?

When you define only function prototype in a base class without implementation and do the complete implementation in derived class. This base class is called abstract class and client won't able to instantiate an object using this base class.

You can make a pure virtual function or abstract class this way..

class Boo
{
public:

virtual void foo() = 0;
}

Boo MyBoo; // compilation error

C++ frequently asked questions (1)

2012-03-24T17:57:00.000-07:00

What is inheritance?
Inheritance allows one class to reuse the state and behavior of another class. The derived class inherits the properties and method implementations of the base class and extends it by overriding methods and adding additional properties and methods.

What is encapsulation??
Containing and hiding information about an object, such as internal data structures and code. Encapsulation isolates the internal complexity of an object's operation from the rest of the application. For example, a client component asking for net revenue from a business object need not know the data's origin.

What is Polymorphism??
Polymorphism allows a client to treat different objects in the same way even if they were created from different classes and exhibit different behaviors.
You can use implementation inheritance to achieve polymorphism in languages such as C++ and Java.
Base class object's pointer can invoke methods in derived class objects.
You can also achieve polymorphism in C++ by function overloading and operator overloading.

What is assignment operator?
Default assignment operator handles assigning one object to another of the same class. Member to member copy (shallow copy)

What are all the implicit member functions of the class? Or what are all the functions which compiler implements for us if we don't define one.??
default ctor(Constructor)
copy ctor
assignment operator
default destructor
address operator

What is constructor or ctor?
Constructor creates an object and initializes it. It also creates vtable for virtual functions. It is different from other methods in a class.

What is default constructor?
Constructor with no arguments or all the arguments has default values.

What is copy constructor?
Constructor which initializes the it's object member variables ( by shallow copying) with another object of the same class. If you don't implement one in your class then compiler implements one for you.
for example:
Boo Obj1(10); // calling Boo constructor
Boo Obj2(Obj1); // calling boo copy constructor
Boo Obj2 = Obj1;// calling boo copy constructor

When are copy constructors called?
Copy constructors are called in following cases:
a) when a function returns an object of that class by value
b) when the object of that class is passed by value as an argument to a function
c) when you construct an object based on another object of the same class
d) When compiler generates a temporary object

What is conversion constructor?
constructor with a single argument makes that constructor as conversion ctor and it can be used for type conversion.
for example:
class Boo
{
public:
Boo( int i );
};
Boo BooObject = 10 ; // assigning int 10 Boo object

What is destructor?
Destructor usually deletes any extra resources allocated by the object.

What is conversion operator??
class can have a public method for specific data type conversions.
for example:
class Boo
{
double value;
public:
Boo(int i )
operator double()
{
return value;
}
};
Boo BooObject;
double i = BooObject; // assigning object to variable i of type double. now conversion operator gets called to assign the value.

Check elapsed time

2012-03-24T17:53:00.000-07:00

check elapsed time

Here I collected several ways to check elapsed time of agiven PID, when you didn’t have time or date added to the command line

1) ps –oetimePID

2) ps –p PID–o etime=

3) check thetime when the output file is updated

This could be used to check the elapsed time even if the jobis done, as long as you have a start time

4) ps -eopid, etime, args

5) A shellscript with a bit of perl (should work on all variants of UNIX)

#!/bin/ksh

#puptime.ksh - Gavin Satur - http://cybergav.in/2009/11/09/puptime

#Simple script to calculate the uptime (elapsed wall clock time) of a process

##############################################################################

#

#Accept PID and do some basic validation

#

proc_pid=$1

if [ -z "$proc_pid" ]; then

print "\nERROR : Missing inputargument. SYNTAX: ksh puptime.ksh <pid>\n"

exit 999

fi

if [ ! -d /proc/$proc_pid ]; then

print "\nERROR : No directory forPID $proc_pidin /proc. Check if process is running!\n"

exit 999

fi

#

#Calculate start time of process and current time in epoch time using a bit ofperl

#

proc_stime=`perl -e 'use File::stat; my $filename= "$ARGV[0]"; $sb = stat($filename); printf "%s",$sb->mtime;' /proc/$proc_pid`

currtime=`perl -e 'print time;'`

#

#Calculate process uptime in seconds and then slice'n'dice for human-friendlyoutput

#

proc_time=$(( currtime - proc_stime ))

proc_time_days=$(( proc_time / 86400 ))

proc_time_secs=$(( proc_time % 86400 ))

proc_time_hours=$(( proc_time_secs / 3600 ))

proc_time_secs=$(( proc_time_secs % 3600 ))

proc_time_minutes=$((proc_time_secs / 60 ))

proc_time_secs=$(( proc_time_secs % 60 ))

print"\nUPTIME FOR PROCESS WITH PID $proc_pid = $proc_time_days day(s) $proc_time_hours hour(s) $proc_time_minutes minute(s) $proc_time_secs second(s) \n"

Here we recommend 1) and 4) . It prints out the elapsed time(the “e” in etime) in POSIX format. The output format is like this:

5059 7-13:09:53 /bin/zsh /usr/bin/grml-x fluxbox

…which of course means: PID 5059, elapsed time 7 days, 13hours,09 minutes, 53 seconds.

This is the time I started this cute little window managerfrom the shell.

The ps parameters used are :

-e lists information about evey process running

-o prints information according to the format specified

…so saying “I want the pid, the elapsed time, and all therest of the info” gives you the result up above.

There is also an option stime for -o , but this only returnsthe starting day.

techero@barrioverde ~ % ps -eo pid,stime,args|grep 5059

5059 Feb07 /bin/zsh /usr/bin/grml-x fluxbox

Difference between c and c++

2011-09-19T12:43:00.000-07:00

Difference between c and c++

1. C does not have any classes or objects. It is procedure and function driven. There is no concept of access through objects and structures are the only place where there is a access through a compacted variable. c++ is object oriented.

2. C structures have a diferent behaviour compared to c++ structures. Structures in c do not accept functions as their parts.

3. C input/output is based on library and the prcesses are carried out by including functions. C++ i/o is made through console commands cin and cout.

4. C functions do not support overloading. Operator overloading is a process in which the same function has two or more different behaviours based on the data input by the user.

5. C does not support new or delete commands. The memory operations to free or alllocate memory in c are carried out by malloc() and free().

6. Undeclared functions in c++ are not allowed. The function has to have a prototype defined before the main() before use in c++ although in c the functions can be declared at the point of use.

7. After declaring structures and enumerators in c we cannot declare the variable for the structure right after the end of the structure as in c++.

8. For an int main() in c++ we may not write a return statement but the return is mandatory in c if we are using int main().

9. In C++ identifiers are not allowed to contain two or more consecutive underscores in any position. C identifiers cannot start with two or more consecutive underscores, but may contain them in other positions.

10. C has a top down approch whereas c++ has a bottom up approach.

11. in c a character constant is automatically elevated to and integer whereas in c++ this is not the case.

12. In c declaring the global variable several times is allowed but this is not allowed in c++.

Source: here.

Source for Data Structure Interviews

2011-09-09T19:52:00.000-07:00

Facebook /Mircosoft interview question - Analysis + several solutions in C++/Java

2011-08-21T10:55:00.000-07:00

Given an array A, output another array B such that B[k] is the product of all elements in A but A[k]. Division is not allowed, More specifically,

Given an array of numbers, nums, return an array of numbers products, where products[i] is the product of all nums[j], j != i.
Input : [1, 2, 3, 4, 5]
Output: [(2*3*4*5), (1*3*4*5), (1*2*4*5), (1*2*3*5), (1*2*3*4)]
      = [120, 60, 40, 30, 24]
You must do this in O(N) without using division.

Full analysis:

An explanation of the method is: The trick is to construct the arrays (in the case for 4 elements)

{              1,         a[0],    a[0]*a[1],    a[0]*a[1]*a[2],  }
{ a[1]*a[2]*a[3],    a[2]*a[3],         a[3],                 1,  }

Both of which can be done in O(n) by starting at the left and right edges respectively.
Then multiplying the two arrays element by element gives the required result
My code would look something like this:

int a[N] // This is the input
int products_below[N];
p=1;
for(int i=0;i<N;++i)
{
  products_below[i]=p;
  p*=a[i];
}

int products_above[N];
p=1;
for(int i=N-1;i>=0;--i)
{
  products_above[i]=p;
  p*=a[i];
}

int products[N]; // This is the result
for(int i=0;i<N;++i)
{
  products[i]=products_below[i]*products_above[i];
}

If you need to be O(1) in space too you can do this (which is less clear IMHO)

int a[N] // This is the input
int products[N];

// Get the products below the curent index
p=1;
for(int i=0;i<N;++i)
{
  products[i]=p;
  p*=a[i];
}

// Get the products above the curent index
p=1;
for(int i=N-1;i>=0;--i)
{
  products[i]*=p;
  p*=a[i];
}

Here is a small recursive function in C++ to do the modification in place. It requires O(n) extra space (on stack) though. Assuming the array is in a and N holds the array length, we have

int multiply(int *a, int fwdProduct, int indx) {
    int revProduct = 1;
    if (indx < N) {
       revProduct = multiply(a, fwdProduct*a[indx], indx+1);
       int cur = a[indx];
       a[indx] = fwdProduct * revProduct;
       revProduct *= cur;
    }
    return revProduct;
}

//Here is a small recursive function in Java

static int multiply(int[] nums, int p, int n) {
    return (n == nums.length) ? 1
      : nums[n] * (p = multiply(nums, nums[n] * (nums[n] = p), n + 1))
          + 0*(nums[n] *= p);
}

int[] arr = {1,2,3,4,5};
multiply(arr, 1, 0);
System.out.println(Arrays.toString(arr));
// prints "[120, 60, 40, 30, 24]"

Here's another method to solve it in Java. Apologies for the non-standard formatting, but the code has a lot of duplication, and this is the best I can do to make it readable.

import java.util.Arrays;

public class Products {
    static int[] products(int... nums) {
        final int N = nums.length;
        int[] prods = new int[N];
        Arrays.fill(prods, 1);
        for (int
           i = 0, pi = 1    ,  j = N-1, pj = 1  ;
           (i < N)         && (j >= 0)          ;
           pi *= nums[i++]  ,  pj *= nums[j--]  )
        {
           prods[i] *= pi   ;  prods[j] *= pj   ;
        }
        return prods;
    }
    public static void main(String[] args) {
        System.out.println(
            Arrays.toString(products(1, 2, 3, 4, 5))
        ); // prints "[120, 60, 40, 30, 24]"
    }
}

//Source: here

Morgan Stanley Interview Question

2011-08-17T20:10:00.000-07:00

Morgan Stanley Interview Question: 2010

Suppose you have a large file with lots of words. How would you find the unique words and their count?
What kind of data structure u will use? What will be the time complexity and space complexity?

/* Algorithm to count words in a file. Please dont reply opinions on the code, comments on complexity analysis is appreciated
For each word check if the word exists in the hash, if it doesnt then insert the word with a count 1
if it does then increment the value
For each word in the hashmap, print word and its count
Complexity: Linear insertion and space
NOTE:The code only compiles with newer C++ use the switch -std=c++0x in your Makefile*/
#include <iostream>
#include <vector>
#include <unordered_map>
typedef std::unordered_map<std::string, int> MyHashMap;
void printMyHashMap(MyHashMap::const_iterator itr,MyHashMap::const_iterator enditr){
while(itr!=enditr){
std::cout<<(*itr).first<<" : "<<(*itr).second<<std::endl;
++itr;
}
}
int main(){
MyHashMap wordsMap; //declare unordered map
//vector of strings
std::vector<std::string> strings {"count","words","in","a","file",
"and","print","words","with","their","count",
".","more","words","file"};
int len = strings.size();
std::vector<std::string>::const_iterator wordsItr;
wordsItr = strings.begin();
MyHashMap::iterator mapItr = wordsMap.begin();
for ( wordsItr=strings.begin(); wordsItr!=strings.end(); wordsItr++ ){
mapItr = wordsMap.find(*wordsItr);
if(mapItr==wordsMap.end()){ //if word not found then
wordsMap.insert(MyHashMap::value_type(*wordsItr, 1));
}else{
(*mapItr).second = ++(*mapItr).second; }
}
std::cout<<"words and their count :"<<std::endl;
printMyHashMap(wordsMap.begin(),wordsMap.end());
return (0);
}

///2

#include <iostream>
#include <algorithm>
#include <vector>
#include <fstream>
#include <map>
#include <iterator>
using namespace std;
int main() {
string str;
ifstream in("input.txt");
map<string, int> m;
if (in.is_open()) {
while(in >> str) {
m[str]++;
}
}
for(map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
cout << (*it).first <<"\t" << (*it).second << endl;
}
}
input.txt
abc abc abc abc abc abc abc abcdef abc abcd
abc abc abc abc abc abc abc abcdef abc abcd
abc abc abc abc abc abc abc abcdef abc abcd
abc abc abc abc abc abc abc abcdef abc abcd
count words in a file and print words with their count more words file
output
ajay@ubuntu:~/workspace$ ./a.out
a 1
abc 32
abcd 4
abcdef 4
and 1
count 2
file 2
in 1
more 1
print 1
their 1
with 1
words 3

//Java method

import java.util.HashMap;
public class TestTest {
public static void main(String[] args){
String inputString = "abc abc abc abc abc abc abc abcdef abc abcd abc abc abc abc abc abc abc abcdef abc abcd" +
" abc abc abc abc abc abc abc abcdef abc abcd abc abc abc abc abc abc abc abcdef abc abcd" +
" count words in a file and print words with their count more words file";
HashMap<Object, Integer> myMap = new HashMap<Object, Integer>(){
@Override
public Integer put(Object arg0, Integer arg1) {
Integer i = super.get(arg0);
arg1 = i==null?1:i+1;
return super.put(arg0, arg1);
}
};

String[] splits = inputString.split(" ");
for(String s : splits){
myMap.put(s,1);
}

System.out.println(myMap);
}
}

Source: Here

Interview Questions: wap to print "hello world" without reaching the main function in C++

2011-08-17T19:49:00.000-07:00

There are several ways to do this:

1.

#include<stdio.h>
#include<conio.h>
#define uttam main
uttam()
{
printf("hello world");
getch();
return 0;
}

2.

#include<stdio.h>
#define(s,t,u,m,p,e,d)m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf("Hello World");
}

3.

#include<stdio.h>
#include<iostream>

class World
{
 public:
 World()
 {
  printf("Hello world\n");
 }
};

World w1;

int main()
{
	printf("Hello world again!!\n");
	return 0;
}

source, click here.

UNIX command Questions Answers asked in Interview

2011-08-17T19:44:00.000-07:00

UNIX command Questions Answers asked in Interview

UNIX or Linux operating system has become default Server operating system and for whichever programming job you give interview you find some UNIX command interview questions there. These UNIX command interview questions are mostly asked during Java development and Support role interviews on various investment banks mostly because most of electronic trading systems or stock trading system works on Unix servers. As we know that high volume low latency systems which wants to take advantage of little bit of volatility in market for Equity , Futures and options or Foreign exchange trading need a stable server side operating system and Redhat Linux is doing great job there. with the advent of Algorithmic trading this speed factor becomes more important so getting someone who has good knowledge of operating system and commands on which these trading system runs is definitely required. but these UNIX command interview questions are equally applicable for any job interview which requires some work on Unix Operating System. With the growing use of Linux in form of RedHat, Solaris and IBM AIX its must to keep you familiar with essential Linux commands available on various platforms.

Long back I had once asked one of my friend why are you preparing Unix Command interview questions if you going for a Java Interview and he told me that this job doesn't only require knowledge of Java but also knowledge of Unix, Linux, SQL and other scripting language , which is quite true. After that I thought to collect various UNIX command interview questions asked to Java developers or trading system support interviews and this is the result of that compilation. This list of UNIX command interview questions are by means complete and would be great if you guys contribute some genuine and good Unix Command Interview questions and answers asked during interviews. I have divided the questions on three categories for sake of managing and keeping this list of Unix Interview questions up to date.

Beginners UNIX Interview Questions Answers

1. Write command to list all the links from a directory?

In this UNIX command interview questions interviewer is generally checking whether user knows basic use of "ls" "grep" and regular expression etc

You can write command like:

ls -lrt | grep "^l"

2. Create a read-only file in your home directory?

This is a simple UNIX command interview questions where you need to create a file and change its parameter to read-only by using chmod command you can also change your umask to create read only file.

touch file

chmod 400 file

3. How will you find which operating system your system is running on in UNIX?

By using command "uname -a" in UNIX

4. How will you run a process in background? How will you bring that into foreground and how will you kill that process?

For running a process in background use "&" in command line. For bringing it back in foreground use command "fg jobid" and for getting job id you use command "jobs", for killing that process find PID and use kill -9 PID command. This is indeed a good Unix Command interview questions because many of programmer not familiar with background process in UNIX.

5. How do you know if a remote host is alive or not?

You can check these by using either ping or telnet command in UNIX. This question is most asked in various Unix command Interview because its most basic networking test anybody wants to do it.

6. How do you see command line history in UNIX?

Very useful indeed, use history command along with grep command in unix to find any relevant command you have already executed. Purpose of this Unix Command Interview Questions is probably to check how familiar candidate is from available tools in UNIX operation system.

7. How do you copy file from one host to other?

Many options but you can say by using "scp" command. You can also use rsync command to answer this UNIX interview question or even sftp would be ok.

8. How do you find which process is taking how much CPU?

By using "top" command in UNIX, there could be multiple follow-up UNIX command interview questions based upon response of this because “TOP” command has various interactive options to sort result based upon various parameter.

9. How do you check how much space left in current drive ?

By using "df" command in UNIX. For example "df -h ." will list how full your current drive is. This is part of anyone day to day activity so I think this Unix Interview question will be to check anyone who claims to working in UNIX but not really working on it.

10. What is the difference between Swapping and Paging?

Swapping:

Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging systems.

Paging:

Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the virtual memory. It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more flexibly.

Intermediate UNIX Interview Questions Answers

1. What is difference between ps -ef and ps -auxwww?

This is indeed a good Unix Interview Command Question and I have faced this issue while ago where one culprit process was not visible by execute ps –ef command and we are wondering which process is holding the file.

ps -ef will omit process with very long command line while ps -auxwww will list those process as well.

2. How do you find how many cpu are in your system and there details?

By looking into file /etc/cpuinfo for example you can use below command:

cat /proc/cpuinfo

3. What is difference between HardLink and SoftLink in UNIX?

I have discussed this Unix Command Interview questions in my blog post difference between Soft link and Hard link in Unix

4. What is Zombie process in UNIX? How do you find Zombie process in UNIX?

When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it - for example, the parent may need to check the child's exit status. To be able to get this information, the parent calls 'wait()'; In the interval between the child terminating and the parent calling 'wait()', the child is said to be a 'zombie' (If you do 'ps', the child will have a 'Z' in its status field to indicate this.)

Zombie : The process is dead but have not been removed from the process table.

5. What is "chmod" command? What do you understand by this line “r-- -w- --x?

6. There is a file some where in your system which contains word "UnixCommandInterviewQuestions” How will find that file in Unix?

By using find command in UNIX for details see here 10 example of using find command in Unix

7. In a file word UNIX is appearing many times? How will you count number?

grep -c "Unix" filename

8. How do you set environment variable which will be accessible form sub shell?

By using export for example export count=1 will be available on all sub shell.

9. How do you check if a particular process is listening on a particular port on remote host?

By using telnet command for example “telnet hostname port”, if it able to successfully connect then some process is listening on that port. To read more about telnet read networking command in UNIX

10. How do you find whether your system is 32 bit or 64 bit ?

Either by using "uname -a" command or by using "arch" command.

Advanced UNIX Interview Questions and Answers

1. How do you find which processes are using a particular file?

By using lsof command in UNIX. It wills list down PID of all the process which is using a particular file.

2. How do you find which remote hosts are connecting to your host on a particular port say 10123?

By using netstat command execute netstat -a | grep "port" and it will list the entire host which is connected to this host on port 10123.

3. What is nohup in UNIX?

4. What is ephemeral port in UNIX?

Ephemeral ports are port used by Operating system for client sockets. There is a specific range on which OS can open any port specified by ephemeral port range.

5. If one process is inserting data into your MySQL database? How will you check how many rows inserted into every second?

Purpose of this Unix Command Interview is asking about "watch" command in UNIX which is repeatedly execute command provided with specified delay.

6. There is a file Unix_Test.txt which contains words Unix, how will you replace all Unix to UNIX?

You can answer this Unix Command Interview question by using SED command in UNIX for example you can execute sed s/Unix/UNIX/g fileName.

7. You have a tab separated file which contains Name, Address and Phone Number, list down all Phone Number without there name and Addresses?

To answer this Unix Command Interview question you can either you AWK or CUT command here. CUT use tab as default separator so you can use

cut -f3 filename.

8. Your application home directory is full? How will you find which directory is taking how much space?

By using disk usage (DU) command in Unix for example du –sh . | grep G will list down all the directory which has GIGS in Size.

9. How do you find for how many days your Server is up?

By using uptime command in UNIX

10. You have an IP address in your network how will you find hostname and vice versa?

This is a standard UNIX command interview question asked by everybody and I guess everybody knows its answer as well.

By using nslookup command in UNIX, you can read more about networking command in UNIX here.

Source: click here.

Insight of OOP Object -- How inheritance, encapsulation and polymorphism work

2011-06-24T18:50:00.000-07:00

How inheritance, encapsulation and polymorphism work in C++

alexonlinux.com

Introduction
Encapsulation
How methods work
How overloading works
How mangling solves the problem
Structure and size of the object
How inheritance and polymorphism work
How basic polymorphism works
How multiple inheritance works
Difference between different casting types
Polymorphism and multiple inheritance
What if we try something even more complicated
Few words about C++ constructors
Conclusion

IntroductionBACK TO TOC

Inheritance, encapsulation and polymorphism are undoubtedly the cornerstones of OOP/OOD in general and C++ in particular.
When programming C, it is very easy to remember how things work. You know that when you add an int variable to a structure it mostly grows by four bytes. You know that long is either four or eight bytes long depending on the architecture you’re working with.
Things are less obvious when moving to C++. OOP brings more abstractions to the program. As a result you are no longer sure if a+b sums two numbers or calls some overloaded operator method that concatenates contents of two files together.
In this article, I would like to give you a short insight into what’s going on behind the scenes. In particular we’ll see how the three whales of OOP work in C++.
Things that I am going to show in this article may differ from compiler to compiler. I will talk mostly about g++ (version 4.2.3). Note however, that same ideas apply everywhere.

EncapsulationBACK TO TOC

As you know, encapsulation is a principle by which same entity, the object, encapsulates data and methods that manipulate the data. You may be surprised to find out that underneath, class methods are just plain functions.

How methods workBACK TO TOC

In C++ there’s one fundamental difference between plain functions and class methods. Class methods receive one additional argument and that is the pointer to the object whose data the method is expected to manipulate. I.e. first argument to a method is pointer to this.
To speed things up, C++ developers used single CPU register (ECX/RCX on x86/x86_64) to pass pointer to this, instead of passing it via stack as if it was a regular function argument (no longer true in x86_64).
Otherwise, objects know nothing about methods that operate on them.

How overloading worksBACK TO TOC

Another thing that we have to take care of in C++ is how to distinguish between some_function() and some_class::some_function(). Or between some_class::some_function( int ) and some_class::some_function() I.e. what’s the difference between two methods with the same name that receive different number and type of arguments? What is the difference between method and function that has same name?
Obviously, out of linker, compiler and preprocessor, linker is the one that should be aware of the above difference. This is because we may have some_function() in some distant object file. Linker is the component that should find this distant function and interconnect the call to the function and the actual function. Linker uses function name as a unique identifier of the function.
To make things work, g++ and any other modern compiler, mangles the name of the method/function and makes sure that:

Mangled method name includes name of the class it belongs to (if it belongs to any class).
Mangled method name includes number and type of arguments method receives.
Mangled method name includes namespace it belongs to.

With these three, some_class::some_function() and some_function() will have totally different mangled name. See the following code sample.

01namespace some_namespace
02{
03    class some_class
04    {
05    public:
06        some_class() { }
07        void some_method() { }
08    };
09};
10 
11class some_class
12{
13public:
14    some_class() { }
15    void some_method() { }
16};
17 
18void some_method()
19{
20    int a;
21}

g++ will turn:

void some_class::some_method() into _ZN10some_class11some_methodEv
void some_namespace::some_class::some_method() into _ZN14some_namespace10some_class11some_methodEv
void some_method() into _Z11some_methodv

Adding integer argument to void some_method() will turn it from _Z11some_methodv to _Z11some_methodi.

How mangling solves the problemBACK TO TOC

So when you create two methods with same name, but with different arguments, compiler turns them into two functions with different names. Later, when linker links the code together it doesn’t know that these are two methods of the same class. From linkers standpoint, these are two different functions.

Structure and size of the objectBACK TO TOC

You probably already know that C++ class and good old C structures are nearly the same thing. Perhaps the only difference is that all class members are private unless specified otherwise. On the contrary, all structure members are public.
When looking at the memory layout of the object, it is very similar to C structure.

Differences begin when you add virtual methods. Once you add virtual methods to the class, compiler will create virtual methods table for the class. Then it will place pointer to the table in the beginning of each instance of this class.

So, bear in mind that once your class has virtual methods, each object of this class will be four or eight bytes (depends on whether you have 64-bit support or not) bigger.
Actually, pointer to the virtual methods table does not have to be at the beginning of the object. It is just handy to keep it at the beginning, so g++ and most of the modern compilers do it this way.
Adding virtual methods to the class will also increase amount of RAM your program consumes and its size on your hard drive.

How inheritance and polymorphism workBACK TO TOC

Lets say we have two classes. A and B. Class B inherits from class A.

01#include <iostream>
02 
03using namespace std;
04 
05class A
06{
07public:
08    A() { a_member = 0; }
09    int a_member;
10};
11 
12class B : public A
13{
14public:
15    B() : A() { b_member = 0; };
16    int b_member;
17};
18 
19int main()
20{
21    A *a = new B;
22    a->a_member = 10;
23 
24    return 0;
25}

The interesting thing to notice here is that a actually points to instance of class B. When dereferencing a_member, we’re actually dereferencing a_member that defined in class A, but belongs to class B (via inheritance). To make this happen, compiler has to make sure that common part of both classes (a_member in our case) located at the same offset in the object.

Now what if we have some virtual methods.

How basic polymorphism worksBACK TO TOC

Let’s change our example a bit and add some virtual methods.

01#include <iostream>
02 
03using namespace std;
04 
05class A
06{
07public:
08    A() { a_member = 0; }
09    virtual int reset() { a_member = 0; }
10    void set_a_member( int a ) { a_member = a; }
11    int get_a_member() { return a_member; }
12protected:
13    int a_member;
14};
15 
16class B : public A
17{
18public:
19    B() : A() { b_member = 0; };
20    virtual int reset() { a_member = b_member = 0; }
21    virtual void some_virtual_method() { }
22    void set_b_member(int b ) { b_member = b; }
23    int get_b_member() { return b_member; }
24protected:
25    int b_member;
26};
27 
28int main()
29{
30    B *b = new B;
31    A *a = b;
32 
33    b->set_b_member( 20 );
34    b->set_a_member( 10 );
35 
36    a->reset();
37 
38    cout << b->get_a_member() << " " << b->get_b_member() <<
39        endl;
40 
41    return 0;
42}

If you compile and run this program it will obviously print “0 0″. But how, you may ask. After all we did a->reset(). Without our understanding of polymorphism we could think that we’re calling method that belongs to class A.
The reason it works is because when compiler sees code that dereferences pointer to A it expects certain internal object structure and when it dereferences pointer to B it expects different object structure. Let us take a look at both of them.

However even more important here is the structure of the virtual methods tables of both classes.

It is because of the virtual methods table structure compilers knows what virtual method to call. When it generates the code that dereferences pointer to A, it expects that first method in the virtual methods table of the object will be pointer to right reset() routine. It doesn’t really care if the pointer actually points to B object. It will call first method of the virtual methods table anyway.

How multiple inheritance worksBACK TO TOC

Multiple inheritance makes things much more complicated. The problem is that when class C inherits from both A and B, we should have both members of class A and class B in the instance of class C.

01#include <iostream>
02 
03using namespace std;
04 
05class A
06{
07public:
08    A() { a_member = 0; }
09protected:
10    int a_member;
11};
12 
13class B
14{
15public:
16    B() { b_member = 0; }
17protected:
18    int b_member;
19};
20 
21class C : public A, public B
22{
23public:
24    C() : A(), B() { c_member = 0; }
25protected:
26    int c_member;
27};
28 
29int main()
30{
31    C c;
32 
33    A *a1 = &c;
34    B *b1 = &c;
35 
36    A *a2 = reinterpret_cast<A *>( &c );
37    B *b2 = reinterpret_cast<B *>( &c );
38 
39    printf( "%p %p %p\n", &c, a1, b1 );
40    printf( "%p %p %p\n", &c, a2, b2 );
41 
42    return 0;
43}

Once we cast pointer to class C into class B, we cannot keep the value of the pointer as is because first fields in the object occupied by fields defined in class A (a_member). Therefore, when we do casting we have to do a very special kind of casting – the one that changes the actual value of the pointer.
If you compile and run above code snippet, you will see that all the values are the same except for b1, which should be 4 bytes bigger than other values.

This is what (C style casting in our case) casting does – it increments the value of the pointer to make sure that it points to the beginning of the, inherited from B, part of the object.
In case you wonder what other types of casting will do, here is a short description.

Difference between different casting typesBACK TO TOC

There are five types of casting in C++.

reinterpret_cast<>()
static_cast<>()
dynamic_cast<>()
const_cast<>()
C style cast.

I guess you know already what const_cast<>() does. Also, it is only a compile time casting. C style cast is same as static_cast<>(). This leaves us with three types of casting.

reinterpret_cast<>()
static_cast<>()
dynamic_cast<>()

From the above example we learn that reinterpret_cast<>() does nothing to the pointer value and leaves it as is.
static_cast<>() and dynamic_cast<>() both modify value of the pointer. The difference between two is that the later relies on RTTI to see if the casting is legal – it looks inside the object to see if it truly belongs to the type we’re trying to cast from. static_cast<>() on the other hand, simply increments the value of the pointer.

Polymorphism and multiple inheritanceBACK TO TOC

Things getting even more complicated when we have virtual methods in each one of the classes A, B and C that we already met. Let’s add following virtual methods to the classes.

virtual void set_a( int new_a ) { a_member = new_a; }

To class A.

virtual void set_b( int new_b ) { b_member = new_b; }

To class B and

virtual void set_c( int new_c ) { c_member = new_c; }

To class C.

You could have assumed that even in this case class C objects will have only one virtual tables methods, but this is not true. When you static_cast class C object into class B object, class B object must have its own virtual tables method. If we want to use same casting method as with regular objects (that is adding few bytes to the pointer to reach right portion of the object), then we have no choice but to place another virtual tables method in the middle of the object.

As a result, you can have many different virtual methods tables for the same class. The above diagram shows very simple case of inheritance and the truth is that it does not get more complicated than this. Take a look at the following, more complex, class hierarchy.

It may surprise you, but structure of the class X object will be quiet simple. In our previous example inheritance hierarchy had two branches. This one has three:

A-C-F-X
D-G-X
B-E-H-X

All end up with X of course. They are a little longer than in our previous example, but there is nothing special about them. The structure of the object will be the following:

As a rule of thumb, g++ (and friends) calculates the branches that lead to the target class, class X in our case. Next it creates a virtual methods table for each branch and places all virtual methods from all classes in the branch into virtual methods table. This includes pointer to virtual methods of the class itself.
If we project this rule onto our last example. A-C-F-X branch virtual methods table will include pointers to virtual methods from classes A, C, F and X. Same with other two branches.

What if we try something even more complicatedBACK TO TOC

The thing is that you can’t. Lets say we try to create even more complicated hierarchy by changing class D from our previous example to inherit from class C.

This will immediately create ambiguous inheritance and the compiler will not hesitate to tell you that this is what happened. This is because now class X will have all members of classes A and C twice. Once it will have it via A-C-F-X branch and once via A-C-D-G-X branch. It will not tell you that there’s a problem immediately. Instead, once you will try to reference one of the members of X inherited from either A or C, g++ will tell you that it has two variations of the same member/method and that it does not know which one of them to call.
This what would be g++ output if you try to compile this file.

main.cc: In function 'int main()':
main.cc:110: error: request for member 'set_a' is ambiguous
main.cc:29: error: candidates are: virtual void A::set_a(int)
main.cc:29: error:                 virtual void A::set_a(int)

All this because I was trying to do x.set_a( 20 ); in line 110.

Few words about C++ constructorsBACK TO TOC

I guess you know what constructors are good for. In light of what we’ve seen, you may ask yourself, who is building all those virtual methods tables and who writes right pointer into the object.
Obviously compiler builds all the virtual methods tables. And constructor is the one who fills in the right virtual methods table. And this is another reason why you cannot call constructor directly – you don’t want to mess up with virtual methods tables.

Source: Click here

Java Interview Questions - Basic Questions

2011-06-22T18:36:00.000-07:00

1.What is Constructor?

A constructor is a special method whose task is to initialize the object of its class.
It is special because its name is the same as the class name.
They do not have return types, not even void and therefore they cannot return values.
They cannot be inherited, though a derived class can call the base class constructor.
Constructor is invoked whenever an object of its associated class is created.

2.How does the Java default constructor be provided?

If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

3.Can constructor be inherited?

No, constructor cannot be inherited, though a derived class can call the base class constructor.

4.What are the differences between Contructors and Methods?

	Constructors	Methods
Purpose	Create an instance of a class	Group Java statements
Modifiers	Cannot be abstract, final, native, static, or synchronized	Can be abstract, final, native, static, or synchronized
Return Type	No return type, not even void	void or a valid return type
Name	Same name as the class (first letter is capitalized by convention) -- usually a noun	Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
*this*	Refers to another constructor in the same class. If used, it must be the first line of the constructor	Refers to an instance of the owning class. Cannot be used by static methods.
*super*	Calls the constructor of the parent class. If used, must be the first line of the constructor	Calls an overridden method in the parent class
Inheritance	Constructors are not inherited	Methods are inherited

5.How are this() and super() used with constructors?

Constructors use this to refer to another constructor in the same class with a different parameter list.
Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

6.What are the differences between Class Methods and Instance Methods?

Class Methods	Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class	Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword. Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.	Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an instance of the class.	Instance methods are not declared as static.

7.How are this() and super() used with constructors?

Constructors use this to refer to another constructor in the same class with a different parameter list.
Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

8.What are Access Specifiers?

One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

9.What are Access Specifiers available in Java?

Java offers four access specifiers, listed below in decreasing accessibility:

Public- public classes, methods, and fields can be accessed from everywhere.
Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.

Situation	`public`	`protected`	default	`private`
Accessible to class from same package?	yes	yes	yes	no
Accessible to class from different package?	yes	no, unless it is a subclass	no	no

10.What is final modifier?

The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

final Classes- A final class cannot have subclasses.
final Variables- A final variable cannot be changed once it is initialized.
final Methods- A final method cannot be overridden by subclasses.

11.What are the uses of final method?

There are two reasons for marking a method as final:

Disallowing subclasses to change the meaning of the method.
Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.

12.What is static block?

Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

13.What are static variables?

Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.

static type  varIdentifier;

where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

14.What is the difference between static and non-static variables?

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

15.What are static methods?

Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:

A static method can only call other static methods.
A static method must only access static data.
A static method cannot reference to the current object using keywords super or this.

Source: Developersbook.com Click here.

OOP interview questions

2011-06-22T18:20:00.000-07:00

(www.developersbook.com) Source: Click here.
1.What are the principle concepts of OOPS?

There are four principle concepts upon which object oriented design and programming rest. They are:

    Abstraction
    Polymorphism
    Inheritance
    Encapsulation
    (i.e. easily remembered as A-PIE).

2.What is Abstraction?

Abstraction refers to the act of representing essential features without including the background details or explanations.

3.What is Encapsulation?

Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

4.What is the difference between abstraction and encapsulation?

    Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
    Abstraction solves the problem in the design side while Encapsulation is the Implementation.
    Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.

5.What is Inheritance?

    Inheritance is the process by which objects of one class acquire the properties of objects of another class.
    A class that is inherited is called a superclass.
    The class that does the inheriting is called a subclass.
    Inheritance is done by using the keyword extends.
    The two most common reasons to use inheritance are:
        To promote code reuse
        To use polymorphism

6.What is Polymorphism?

Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

7.How does Java implement polymorphism?

(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.

    In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
    In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

8.Explain the different forms of Polymorphism.

There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

    Method overloading
    Method overriding through inheritance
    Method overriding through the Java interface

9.What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

10.What is Dynamic Binding?

Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

11.What is method overloading?

Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:

    Overloaded methods MUST change the argument list
    Overloaded methods CAN change the return type
    Overloaded methods CAN change the access modifier
    Overloaded methods CAN declare new or broader checked exceptions
    A method can be overloaded in the same class or in a subclass

12.What is method overriding?

Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:

    The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
    You cannot override a method marked final
    You cannot override a method marked static
    13.What are the differences between method overloading and method overriding?

         Overloaded Method    Overridden Method
    Arguments

    Must change


    Must not change

    Return type


    Can change


    Can’t change except for covariant returns

    Exceptions


    Can change


    Can reduce or eliminate. Must not throw new or broader checked exceptions

    Access


    Can change


    Must not make more restrictive (can be less restrictive)

    Invocation


    Reference type determines which overloaded version is selected. Happens at compile time.


    Object type determines which method is selected. Happens at runtime.

    14.Can overloaded methods be override too?

    Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

    15.Is it possible to override the main method?

    NO, because main is a static method. A static method can't be overridden in Java.

    16.How to invoke a superclass version of an Overridden method?

    To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method.

       // From subclass
           super.overriddenMethod();

    17.What is super?

    super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword.
    Note:
    You can only go back one level.
    In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

18.How do you prevent a method from being overridden?

To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                           public final void exampleMethod() {
                          // Method statements
                          }

19.What is an Interface?

An interface is a description of a set of methods that conforming implementing classes must have.
Note:

    You can’t mark an interface as final.
    Interface variables must be static.
    An Interface cannot extend anything but another interfaces.

20.Can we instantiate an interface?

You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

21.Can we create an object for an interface?

Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

22.Do interfaces have member variables?

Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

23.What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

24.What is a marker interface?

Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

25.What is an abstract class?

Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:

    If even a single method is abstract, the whole class must be declared abstract.
    Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
    You can’t mark a class as both abstract and final.

26.Can we instantiate an abstract class?

An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

27.What are the differences between Interface and Abstract class?

Abstract Class    Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.    An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.    A Class may implement several interfaces.
An abstract class can have non-abstract methods.    All methods of an Interface are abstract.
An abstract class can have instance variables.    An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.    An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.    If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .    An Interface cannot contain constructors .
Abstract classes are fast.    Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

28.When should I use abstract classes and when should I use interfaces?

Use Interfaces when…

    You see that something in your design will change frequently.
    If various implementations only share method signatures then it is better to use Interfaces.
    you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when…

    If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
    When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
    Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

29.When you declare a method as abstract, can other nonabstract methods access it?

Yes, other nonabstract methods can access a method that you declare as abstract.

30.Can there be an abstract class with no abstract methods in it?

Yes, there can be an abstract class without abstract methods.

Code Interview

Linear time algorithm for finding longest palindrome substring

Linear time algorithm for finding

longest palindrome substring

How To Read C Declarations

The golden rule

Function pointers

Combinations

Summary

Java Interview Frequently Asked Questions & Answers (S1P2

Java Interview Frequently Asked Questions & Answers (S1P1)

Java Interview Frequently Asked Questions & Answers (Part Six)

Java Interview Frequently Asked Questions & Answers (Part Five)

Java Interview Frequently Asked Questions & Answers (Part Four)

Java Interview Frequently Asked Questions & Answers (Part Three)

Java Interview Frequently Asked Questions & Answers (Part Two)

Java Interview Frequently Asked Questions & Answers (Part One)

C++ frequently asked questions (4)

C++ frequently asked questions (3) - Pointer and Memory management

C++ frequently asked questions (2)

C++ frequently asked questions (1)

Check elapsed time

Difference between c and c++

Difference between c and c++

Source for Data Structure Interviews

CS 61B Lecture 1 - Course Overview

Top 20 Core Java Interview Questions for Investment Banks

Facebook /Mircosoft interview question - Analysis + several solutions in C++/Java

Morgan Stanley Interview Question

Interview Questions: wap to print "hello world" without reaching the main function in C++

UNIX command Questions Answers asked in Interview

Beginners UNIX Interview Questions Answers

Intermediate UNIX Interview Questions Answers

Advanced UNIX Interview Questions and Answers

Insight of OOP Object -- How inheritance, encapsulation and polymorphism work

How inheritance, encapsulation and polymorphism work in C++

Table of contents

IntroductionBACK TO TOC

EncapsulationBACK TO TOC

How methods workBACK TO TOC

How overloading worksBACK TO TOC

How mangling solves the problemBACK TO TOC

Structure and size of the objectBACK TO TOC

How inheritance and polymorphism workBACK TO TOC

How basic polymorphism worksBACK TO TOC

How multiple inheritance worksBACK TO TOC

Difference between different casting typesBACK TO TOC

Polymorphism and multiple inheritanceBACK TO TOC

What if we try something even more complicatedBACK TO TOC

Few words about C++ constructorsBACK TO TOC

Java Interview Questions - Basic Questions

OOP interview questions