Java FAQs

Difference between String, StringBuffer, StringBuilder

2008-03-28T07:39:00.001-07:00

String: String is immutable. You can't change a string object once after you created it. The methods in String class which appear to be changing string object are actually return a new object.

StringBuffer: StringBuffer is mutable. You can change it without creating new object. Use it when you have to modify your string a lot.

StringBuilder: StringBuilder is introduced in Java SE 5.0. It is same as StringBuffer in functionality. But it doesn't have its methods synchronized, whereas StringBuffer has synchronized methods. When you don't need thread safety for your strings use StringBuilder, it will be little fast as you don't need to get locks unnecessarily. Most of the time you don't need synchronized methods.

Specs work in each Tomcat version

2008-02-07T02:30:00.000-08:00

Different versions of Apache Tomcat are available for different versions of the Servlet and JSP specifications. The mapping between the specifications and the respective Apache Tomcat verisons is:

Servlet/JSP Spec	Apache Tomcat version
2.5/2.1	6.0.x
2.4/2.0	5.5.x
2.3/1.2	4.1.x
2.2/1.1	3.3.x

Loading data from a file into a table

2008-01-17T22:06:00.000-08:00

How to import data from a file to a table

in Oracle
You have to use sqlldr utility to import data from an external file into a table, you can't do from sql plus.

First you have to create a control file, a data file. Control file should be like this

LOAD DATA
INFILE data file name
APPEND INTO TABLE table name
FIELDS TERMINATED BY 'separator'
(columns list)

Separator is the character which separates the column values in the data file.
Ex:
LOAD DATA
INFILE 'C:\data.txt'
APPEND INTO TABLE ac_mt
FIELDS TERMINATED BY ','
(acname, mtname)

Next call the sqlldr from command prompt and give it control file.

Syntax: sqlldr control='control file name' log='log file name' bad='bad file name'
Any rows which are not imported will be placed into bad file. Details will be logged into the log file.

Read more here.

in MySQL
you can give the same command directly or you can use mysqlimport utility.

Read more here.

Anonymous arrays in Java

2008-01-17T21:24:00.000-08:00

Anonymous arrays are similar to anonymous classes in Java. Both don't have a name, so called anonymous. If you need to create an array to use only once, then you can create an anoymous array without assigning it to any variable.

Syntax: new int[] ( 15, 32, 20)

Just define the array needed followed by the keyword new. No need to specify a size, as you are assigning values to the array immediately.

For example, you need to pass an array to a function, and you don't use it anywhere else then you can do as following.

Ex: void fun( grade, new String[] ("Pass", "Fail") )

Interview - 2

2007-12-07T22:27:00.000-08:00

1. What is the output of the following program?

int b=’A’;

System.out.println(b);

A) 65 B) A C) Compile Time Error D) Runtime Error

2. What is the output of the following program?

String a=new String("Reddy");

StringBuffer b=new StringBuffer("Reddy");

System.out.println(a.equals(b));

A) true B) false C) Compile Time Error D) Runtime Error

3. What is the output of the following program?

String a="Reddy";

a.toUpperCase();

System.out.println(a);

A) REDDY B) Reddy C) Compile Time Error D)Runtime Error

4. Which of the following declarations are wrong?

i. final public class test{}

ii. abstract public class test{}

iii. protected abstract class test{}

iv. public static interface test{}

v. final static interface test{}

A) iii & iv B) iv & v C) i & ii & v D) iii & v

Interview - 1

2007-11-25T06:33:00.001-08:00

What is the super class of Exception?

Throwable

What is the main controller component in Struts framework?

ActionServlet

What is a marker Interface?

Marker Interface is an empty interface which is used to indicate something to the compiler. For example if you are implementing 'Serializable' marker interface you are indicating that the object can be converted into bytes.

Can you call doPost method from doGet ?

yes

2007-11-24T07:53:00.001-08:00

What is the difference between final, finally and finalize?

final: final is a keyword used to create constants and non-inheritable classes. If final is used with a variable, some value should be assigned to that variable, and that value can't be changed again. If final is used for classes, you can't extend those classes.

finally: finally is used with try...catch block. finally is a block of statements which executes nevertheless the exception occurred or not. If exception occurs execution will come to catch block, executes the necessary statements then comes to finally block. If exception doesn't occur then try block will be executed fully, then execution comes to finally block. This finally block was commonly used to cleanup any resources held.

finalize: finalize is a method which will be called before the object is garbage collected. Garbage collector will call this method once, before it reclaims the memory of the object. So, any cleanup code will be put in the finalize block.

2007-11-21T04:58:00.001-08:00

Can we serialize the static variables?

No. Since serialization is the process of converting object's state into bytes so that the object can be sent across network or stored to the physical storage media (like hard disk) for later retrieval. Since static variables are not associated with any object (specific instance), they can't be serialized.

So, serialization is associated with objects, static members are associated with class not an object.

2007-10-23T09:20:00.001-07:00

What is the Difference between Checked and Unchecked exceptions?

Checked exceptions are instances of Exception class or its subclasses. Unchecked exceptions are instances of either Error class, Runtime Exception or its subclasses. We have to catch and handle the checked exceptions, or declare that we can't handle that exception. Compiler will check that we have done one of the two things. So, they are called checked exceptions.

The reason for this is that Errors are occur by conditions outside of the application. and Runtime Exceptions are occur by wrong programming logic. We can't handle these exceptional conditions without stopping the application. So, compiler will not ask you to catch or declare these types of exceptions, even though you can choose to do catch or declare.

Even when you are creating your own exception classes, you have to remember the intentions of checked exceptions, and unchecked exception classes.

2007-09-09T03:33:00.001-07:00

What is the difference between JVM, JRE, and JDK?

Java Virtual Machine interprets and executes the java application.
Java Runtime Environment provides JVM with library classes used in the program and other supporting files.
Java Developer Kit provides tools to the developers to develop applications in java.

JVM is responsible for all things like garbage collection, array bounds checking, etc...