mmp669-Android Development

Java Tutorial 26: Annotations

2013-05-20T17:37:00.000+01:00

Annotations

Annotations provide data about a program that is not part of the program itself. They have no direct effect on the operation of the code they annotate.
Annotations have a number of uses, among them:

Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
Compiler-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
Runtime processing — Some annotations are available to be examined at runtime.

Annotations can be applied to a program's declarations of classes, fields, methods, and other program elements.
The annotation appears first, often (by convention) on its own line, and may include elements with named or unnamed values:

@Author(
   name = "Benjamin Franklin",
   date = "3/27/2003"
)
class MyClass() { }

@SuppressWarnings(value = "unchecked")
void myMethod() { }

If there is just one element named "value," then the name may be omitted, as in:

@SuppressWarnings("unchecked")
void myMethod() { }

Also, if an annotation has no elements, the parentheses may be omitted, as in:

@Override
void mySuperMethod() { }

Documentation

Many annotations replace what would otherwise have been comments in code.
Suppose that a software group has traditionally begun the body of every class with comments providing important information:

public class Generation3List extends Generation2List {

   // Author: John Doe
   // Date: 3/17/2002
   // Current revision: 6
   // Last modified: 4/12/2004
   // By: Jane Doe
   // Reviewers: Alice, Bill, Cindy

   // class code goes here

}

To add this same metadata with an annotation, you must first define the annotation type. The syntax for doing this is:

@interface ClassPreamble {
   String author();
   String date();
   int currentRevision() default 1;
   String lastModified() default "N/A";
   String lastModifiedBy() default "N/A";
   // Note use of array
   String[] reviewers();
}

The annotation type definition looks somewhat like an interface definition where the keyword interface is preceded by the @ character (@ = "AT" as in Annotation Type). Annotation types are, in fact, a form of interface, which will be covered in a later lesson. For the moment, you do not need to understand interfaces.
The body of the annotation definition above contains annotation type element declarations, which look a lot like methods. Note that they may define optional default values.
Once the annotation type has been defined, you can use annotations of that type, with the values filled in, like this:

@ClassPreamble (
   author = "John Doe",
   date = "3/17/2002",
   currentRevision = 6,
   lastModified = "4/12/2004",
   lastModifiedBy = "Jane Doe",
   // Note array notation
   reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {

// class code goes here

}

Note: To make the information in @ClassPreamble appear in Javadoc-generated documentation, you must annotate the @ClassPreamble definition itself with the@Documented annotation:

// import this to use @Documented
import java.lang.annotation.*;

@Documented
@interface ClassPreamble {

   // Annotation element definitions
   
}

Annotations Used by the Compiler

There are three annotation types that are predefined by the language specification itself: @Deprecated, @Override, and @SuppressWarnings.
@Deprecated—the @Deprecated annotation indicates that the marked element is deprecated and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the @Deprecated annotation. When an element is deprecated, it should also be documented using the Javadoc @deprecated tag, as shown in the following example. The use of the "@" symbol in both Javadoc comments and in annotations is not coincidental — they are related conceptually. Also, note that the Javadoc tag starts with a lowercase "d" and the annotation starts with an uppercase "D".

   // Javadoc comment follows
    /**
     * @deprecated
     * explanation of why it
     * was deprecated
     */
    @Deprecated
    static void deprecatedMethod() { }
}

@Override—the @Override annotation informs the compiler that the element is meant to override an element declared in a superclass (overriding methods will be discussed in the the lesson titled "Interfaces and Inheritance").

   // mark method as a superclass method
   // that has been overridden
   @Override 
   int overriddenMethod() { }

While it's not required to use this annotation when overriding a method, it helps to prevent errors. If a method marked with @Override fails to correctly override a method in one of its superclasses, the compiler generates an error.
@SuppressWarnings—the @SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate. In the example below, a deprecated method is used and the compiler would normally generate a warning. In this case, however, the annotation causes the warning to be suppressed.

   // use a deprecated method and tell 
   // compiler not to generate a warning
   @SuppressWarnings("deprecation")
    void useDeprecatedMethod() {
        // deprecation warning
        // - suppressed
        objectOne.deprecatedMethod();
    }

Every compiler warning belongs to a category. The Java Language Specification lists two categories: "deprecation" and "unchecked." The "unchecked" warning can occur when interfacing with legacy code written before the advent of generics (discussed in the lesson titled "Generics"). To suppress more than one category of warnings, use the following syntax:

@SuppressWarnings({"unchecked", "deprecation"})

Annotation Processing

The more advanced uses of annotations include writing an annotation processor that can read a Java program and take actions based on its annotations. It might, for example, generate auxiliary source code, relieving the programmer of having to create boilerplate code that always follows predictable patterns. To facilitate this task, release 5.0 of the JDK includes an annotation processing tool, called apt. In release 6 of the JDK, the functionality of apt is a standard part of the Java compiler.
To make annotation information available at runtime, the annotation type itself must be annotated with @Retention(RetentionPolicy.RUNTIME), as follows:

import java.lang.annotation.*; 

@Retention(RetentionPolicy.RUNTIME)
@interface AnnotationForRuntime {

   // Elements that give information
   // for runtime processing
   
}

Source: oracle

Java Tutorial 25: Nested Classes

2013-05-20T17:15:00.004+01:00

Nested Classes

The Java programming language allows you to define a class within another class. Such a class is called a nested class and is illustrated here:

class OuterClass {
    ...
    class NestedClass {
        ...
    }
}

Terminology: Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.

class OuterClass {
    ...
    static class StaticNestedClass {
        ...
    }
    class InnerClass {
        ...
    }
}

A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. As a member of the OuterClass, a nested class can be declared private, public, protected, or package private. (Recall that outer classes can only be declared public or package private.)

Why Use Nested Classes?

There are several compelling reasons for using nested classes, among them:

It is a way of logically grouping classes that are only used in one place.
It increases encapsulation.
Nested classes can lead to more readable and maintainable code.

Logical grouping of classes—If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. Nesting such "helper classes" makes their package more streamlined.
Increased encapsulation—Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared private. By hiding class B within class A, A's members can be declared private and B can access them. In addition, B itself can be hidden from the outside world.
More readable, maintainable code—Nesting small classes within top-level classes places the code closer to where it is used.

Static Nested Classes

As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class — it can use them only through an object reference.

Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.

Static nested classes are accessed using the enclosing class name:

OuterClass.StaticNestedClass

For example, to create an object for the static nested class, use this syntax:

OuterClass.StaticNestedClass nestedObject =
     new OuterClass.StaticNestedClass();

Inner Classes

As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.
Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance. The next figure illustrates this idea.

An Instance of InnerClass Exists Within an Instance of OuterClass

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass.InnerClass innerObject = outerObject.new InnerClass();

Additionally, there are two special kinds of inner classes: local classes and anonymous classes (also called anonymous inner classes). Both of these will be discussed briefly in the next section.

Inner Class Example

To see an inner class in use, let's first consider an array. In the following example, we will create an array, fill it with integer values and then output only values of even indices of the array in ascending order.
The DataStructure class below consists of:

The DataStructure outer class, which includes methods to add an integer onto the array and print out values of even indices of the array.
The InnerEvenIterator inner class, which is similar to a standard Java iterator. Iterators are used to step through a data structure and typically have methods to test for the last element, retrieve the current element, and move to the next element.
A main method that instantiates a DataStructure object (ds) and uses it to fill the arrayOfInts array with integer values (0, 1, 2, 3, etc.), then calls a printEven method to print out values of even indices of arrayOfInts.

public class DataStructure {
    // create an array
    private final static int SIZE = 15;
    private int[] arrayOfInts = new int[SIZE];
    
    public DataStructure() {
        // fill the array with ascending integer values
        for (int i = 0; i < SIZE; i++) {
            arrayOfInts[i] = i;
        }
    }
    
    public void printEven() {
        // print out values of even indices of the array
        InnerEvenIterator iterator = this.new InnerEvenIterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.getNext() + " ");
        }
    }
    
    // inner class implements the Iterator pattern
    private class InnerEvenIterator {
        // start stepping through the array from the beginning
        private int next = 0;
        
        public boolean hasNext() {
            // check if a current element is the last in the array
            return (next <= SIZE - 1);
        }
        
        public int getNext() {
            // record a value of an even index of the array
            int retValue = arrayOfInts[next];
            //get the next even element
            next += 2;
            return retValue;
        }
    }
    
    public static void main(String s[]) {
        // fill the array with integer values and print out only
        // values of even indices
        DataStructure ds = new DataStructure();
        ds.printEven();
    }
}

The output is:

0 2 4 6 8 10 12 14

Note that the InnerEvenIterator class refers directly to the arrayOfInts instance variable of the DataStructure object.
Inner classes can be used to implement helper classes like the one shown in the example above. If you plan on handling user-interface events, you will need to know how to use inner classes because the event-handling mechanism makes extensive use of them.

Local and Anonymous Inner Classes

There are two additional types of inner classes. You can declare an inner class within the body of a method. Such a class is known as a local inner class. You can also declare an inner class within the body of a method without naming it. These classes are known as anonymous inner classes. You will encounter such classes in advanced Java programming.

Modifiers

You can use the same modifiers for inner classes that you use for other members of the outer class. For example, you can use the access specifiers — private, public, and protected— to restrict access to inner classes, just as you do to other class members.

Summary of Nested Classes

A class defined within another class is called a nested class. Like other members of a class, a nested class can be declared static or not. A nonstatic nested class is called an inner class. An instance of an inner class can exist only within an instance of its enclosing class and has access to its enclosing class's members even if they are declared private.
The following table shows the types of nested classes:

Source: Oracle

Java Encapsulation

2013-05-17T17:27:00.002+01:00

Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding.

Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface.

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.

To run the example below, you need to set-up a new project in Eclipse and put both classes under this project.

account class:

public class account {

private String name;

private String address;

private double balance;

public void setName (String n){

name=n;

}

public String getName(){

return name;

}

public void setAddress (String a){

address=a;

}

public String getAddress(){

return address;

}

public void setBalance(double g){

balance=g;

}

public double getBalance (){

return balance;

}

myaccount class

public class myAccount {

public static void main (String [] args){

account myAccount=new account();

myAccount.setName ("Tom");

myAccount.setAddress ("bugaosuni");

myAccount.setBalance (10000000);

System.out.println(myAccount.getName()+"\t"

+myAccount.getAddress()+"\t"

+myAccount.getBalance());

}

The addition of the word private in front of each of the account
class’s field declarations. The word private is a Java keyword. When a
field is declared private, no code outside of the class can make direct reference
to that field.

Instead of referencing myAccount.name, the useaccount program must
call method myAccount.setName or method myAccount.getName. These
methods, setName and getName, are called accessor methods, because they
provide access to the Account class’s name field.

Eclipse has a very nice function that can generate these for you so you don't have to type in all these tedious code. See screen capture below:

Java Tutorial 24: More on Classes-Initializing Fields

2013-05-16T14:47:00.001+01:00

Before you start reading:

A variable declared inside a class but not inside any particular method is a field.

Another name for a field is an instance variable.

As you have seen, you can often provide an initial value for a field in its declaration:

public class BedAndBreakfast {

    // initialize to 10
    public static int capacity = 10;

    // initialize to false
    private boolean full = false;
}

This works well when the initialization value is available and the initialization can be put on one line. However, this form of initialization has limitations because of its simplicity. If initialization requires some logic (for example, error handling or a for loop to fill a complex array), simple assignment is inadequate. Instance variables can be initialized in constructors, where error handling or other logic can be used. To provide the same capability for class variables, the Java programming language includes static initialization blocks.

Note: It is not necessary to declare fields at the beginning of the class definition, although this is the most common practice. It is only necessary that they be declared and initialized before they are used.

Static Initialization Blocks

A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Here is an example:

static {
    // whatever code is needed for initialization goes here
}

A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.
There is an alternative to static blocks — you can write a private static method:

class Whatever {
    public static varType myVar = initializeClassVariable();
        
    private static varType initializeClassVariable() {

        // initialization code goes here
    }
}

The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.

Initializing Instance Members

Normally, you would put code to initialize an instance variable in a constructor. There are two alternatives to using a constructor to initialize instance variables: initializer blocks and final methods.
Initializer blocks for instance variables look just like static initializer blocks, but without the static keyword:

{
    // whatever code is needed for initialization goes here
}

The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.
A final method cannot be overridden in a subclass. This is discussed in the lesson on interfaces and inheritance. Here is an example of using a final method for initializing an instance variable:

class Whatever {
    private varType myVar = initializeInstanceVariable();
        
    protected final varType initializeInstanceVariable() {

        // initialization code goes here
    }
}

This is especially useful if subclasses might want to reuse the initialization method. The method is final because calling non-final methods during instance initialization can cause problems.

Summary of Creating and Using Classes and Objects

A class declaration names the class and encloses the class body between braces. The class name can be preceded by modifiers. The class body contains fields, methods, and constructors for the class. A class uses fields to contain state information and uses methods to implement behavior. Constructors that initialize a new instance of a class use the name of the class and look like methods without a return type.
You control access to classes and members in the same way: by using an access modifier such as public in their declaration.
You specify a class variable or a class method by using the static keyword in the member's declaration. A member that is not declared as static is implicitly an instance member. Class variables are shared by all instances of a class and can be accessed through the class name as well as an instance reference. Instances of a class get their own copy of each instance variable, which must be accessed through an instance reference.
You create an object from a class by using the new operator and a constructor. The new operator returns a reference to the object that was created. You can assign the reference to a variable or use it directly.
Instance variables and methods that are accessible to code outside of the class that they are declared in can be referred to by using a qualified name. The qualified name of an instance variable looks like this:

objectReference.variableName

The qualified name of a method looks like this:

objectReference.methodName(argumentList)

or:

objectReference.methodName()

The garbage collector automatically cleans up unused objects. An object is unused if the program holds no more references to it. You can explicitly drop a reference by setting the variable holding the reference to null.

Source:oracle

Java Tutorial 23: More on Classes-Understanding Instance and Class Members

2013-05-16T14:45:00.002+01:00

In this section, we discuss the use of the static keyword to create fields and methods that belong to the class, rather than to an instance of the class.

Class Variables

When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables. In the case of the Bicycle class, the instance variables are cadence, gear, and speed. Each Bicycle object has its own values for these variables, stored in different memory locations.
Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are calledstatic fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.
For example, suppose you want to create a number of Bicycle objects and assign each a serial number, beginning with 1 for the first object. This ID number is unique to each object and is therefore an instance variable. At the same time, you need a field to keep track of how many Bicycle objects have been created so that you know what ID to assign to the next one. Such a field is not related to any individual object, but to the class as a whole. For this you need a class variable, numberOfBicycles, as follows:

public class Bicycle{
        
    private int cadence;
    private int gear;
    private int speed;
        
    // add an instance variable for
    // the object ID
    private int id;
    
    // add a class variable for the
    // number of Bicycle objects instantiated
    private static int numberOfBicycles = 0;
        ...
}

Class variables are referenced by the class name itself, as in

Bicycle.numberOfBicycles

This makes it clear that they are class variables.

Note: You can also refer to static fields with an object reference like

myBike.numberOfBicycles

but this is discouraged because it does not make it clear that they are class variables.

You can use the Bicycle constructor to set the id instance variable and increment the numberOfBicycles class variable:

public class Bicycle{
        
    private int cadence;
    private int gear;
    private int speed;
    private int id;
    private static int numberOfBicycles = 0;
        
    public Bicycle(int startCadence, int startSpeed, int startGear){
        gear = startGear;
        cadence = startCadence;
        speed = startSpeed;

        // increment number of Bicycles
        // and assign ID number
        id = ++numberOfBicycles;
    }

    // new method to return the ID instance variable
    public int getID() {
        return id;
    }
        ...
}

Class Methods

The Java programming language supports static methods as well as static variables. Static methods, which have the static modifier in their declarations, should be invoked with the class name, without the need for creating an instance of the class, as in

ClassName.methodName(args)

Note: You can also refer to static methods with an object reference like

instanceName.methodName(args)

but this is discouraged because it does not make it clear that they are class methods.

A common use for static methods is to access static fields. For example, we could add a static method to the Bicycle class to access the numberOfBicycles static field:

public static int getNumberOfBicycles() {
    return numberOfBicycles;
}

Not all combinations of instance and class variables and methods are allowed:

Instance methods can access instance variables and instance methods directly.
Instance methods can access class variables and class methods directly.
Class methods can access class variables and class methods directly.
Class methods cannot access instance variables or instance methods directly—they must use an object reference. Also, class methods cannot use the this keyword as there is no instance for this to refer to.

Constants

The static modifier, in combination with the final modifier, is also used to define constants. The final modifier indicates that the value of this field cannot change.
For example, the following variable declaration defines a constant named PI, whose value is an approximation of pi (the ratio of the circumference of a circle to its diameter):

static final double PI = 3.141592653589793;

Constants defined in this way cannot be reassigned, and it is a compile-time error if your program tries to do so. By convention, the names of constant values are spelled in uppercase letters. If the name is composed of more than one word, the words are separated by an underscore (_).

Note: If a primitive type or a string is defined as a constant and the value is known at compile time, the compiler replaces the constant name everywhere in the code with its value. This is called a compile-time constant. If the value of the constant in the outside world changes (for example, if it is legislated that pi actually should be 3.975), you will need to recompile any classes that use this constant to get the current value.

The `Bicycle` Class

After all the modifications made in this section, the Bicycle class is now:

public class Bicycle{
        
    private int cadence;
    private int gear;
    private int speed;
        
    private int id;
    
    private static int numberOfBicycles = 0;

        
    public Bicycle(int startCadence,
                   int startSpeed,
                   int startGear){
        gear = startGear;
        cadence = startCadence;
        speed = startSpeed;

        id = ++numberOfBicycles;
    }

    public int getID() {
        return id;
    }

    public static int getNumberOfBicycles() {
        return numberOfBicycles;
    }

    public int getCadence(){
        return cadence;
    }
        
    public void setCadence(int newValue){
        cadence = newValue;
    }
        
    public int getGear(){
    return gear;
    }
        
    public void setGear(int newValue){
        gear = newValue;
    }
        
    public int getSpeed(){
        return speed;
    }
        
    public void applyBrake(int decrement){
        speed -= decrement;
    }
        
    public void speedUp(int increment){
        speed += increment;
    }
}

Source: oracle

Java Tutorial 22: More on Classes-Controlling Access to Members of a Class

2013-05-16T13:39:00.002+01:00

Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:

At the top level—public, or package-private (no explicit modifier).
At the member level—public, private, protected, or package-private (no explicit modifier).

A class may be declared with the modifier public, in which case that class is visible to all classes everywhere. If a class has no modifier (the default, also known as package-private), it is visible only within its own package (packages are named groups of related classes)

At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected. The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

The following table shows the access to members permitted by each modifier.

The first data column indicates whether the class itself has access to the member defined by the access level. As you can see, a class always has access to its own members. The second column indicates whether classes in the same package as the class (regardless of their parentage) have access to the member. The third column indicates whether subclasses of the class declared outside this package have access to the member. The fourth column indicates whether all classes have access to the member.

Access levels affect you in two ways. First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use. Second, when you write a class, you need to decide what access level every member variable and every method in your class should have.

Let's look at a collection of classes and see how access levels affect visibility. The following figure shows the four classes in this example and how they are related.

Classes and Packages of the Example Used to Illustrate Access Levels

The following table shows where the members of the Alpha class are visible for each of the access modifiers that can be applied to them.

Tips on Choosing an Access Level: If other programmers use your class, you want to ensure that errors from misuse cannot happen. Access levels can help you do this.

Use the most restrictive access level that makes sense for a particular member. Use private unless you have a good reason not to.
Avoid public fields except for constants. (Many of the examples in the tutorial use public fields. This may help to illustrate some points concisely, but is not recommended for production code.) Public fields tend to link you to a particular implementation and limit your flexibility in changing your code.

Source: oracle

Java Tutorial 21: More on Classes-Using the this Keyword

2013-05-16T13:22:00.002+01:00

Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.

Using `this` with a Field

The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter.
For example, the Point class was written like this

public class Point {
    public int x = 0;
    public int y = 0;
        
    //constructor
    public Point(int a, int b) {
        x = a;
        y = b;
    }
}

but it could have been written like this:

public class Point {
    public int x = 0;
    public int y = 0;
        
    //constructor
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Each argument to the constructor shadows one of the object's fields — inside the constructor x is a local copy of the constructor's first argument. To refer to the Point field x, the constructor must use this.x.

Using `this` with a Constructor

From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Here's anotherRectangle class, with a different implementation from the one in the Objects section.

public class Rectangle {
    private int x, y;
    private int width, height;
        
    public Rectangle() {
        this(0, 0, 0, 0);
    }
    public Rectangle(int width, int height) {
        this(0, 0, width, height);
    }
    public Rectangle(int x, int y, int width, int height) {
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
    }
    ...
}

This class contains a set of constructors. Each constructor initializes some or all of the rectangle's member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor calls the four-argument constructor with four 0 values and the two-argument constructor calls the four-argument constructor with two 0 values. As before, the compiler determines which constructor to call, based on the number and the type of arguments.
If present, the invocation of another constructor must be the first line in the constructor.

Source: ORACLE

Java Tutorial 20: More on Classes-Returning a Value from a Method

2012-04-30T14:19:00.001+01:00

Returning a Value from a Method

A method returns to the code that invoked it when it

completes all the statements in the method,

reaches a return statement, or

throws an exception (covered later),

whichever occurs first.

You declare a method's return type in its method declaration. Within the body of the method, you use the return statement to return the value.

Any method declared void doesn't return a value. It does not need to contain a return statement, but it may do so. In such a case, a return statement can be used to branch out of a control flow block and exit the method and is simply used like this:

return;

If you try to return a value from a method that is declared void, you will get a compiler error.

Any method that is not declared void must contain a return statement with a corresponding return value, like this:

return returnValue;

The data type of the return value must match the method's declared return type; you can't return an integer value from a method declared to return a boolean.

The getArea() method in the Rectangle class discussed earlier returns an integer:

// a method for computing the area of the rectangle

public int getArea() {

return width * height;

}

This method returns the integer that the expression width*height evaluates to.

The getArea method returns a primitive type. A method can also return a reference type. For example, in a program to manipulate Bicycle objects, we might have a method like this:

public Bicycle seeWhosFastest(Bicycle myBike, Bicycle yourBike,

Environment env) {

Bicycle fastest;

// code to calculate which bike is faster, given each bike's gear

// and cadence and given the environment (terrain and wind)

return fastest;

}

Returning a Class or Interface

If this section confuses you, skip it and return to it after you have finished the lesson on interfaces and inheritance.

When a method uses a class name as its return type, such as whosFastest does, the class of the type of the returned object must be either a subclass of, or the exact class of, the return type. Suppose that you have a class hierarchy in which ImaginaryNumber is a subclass of java.lang.Number, which is in turn a subclass of Object, as illustrated in the following figure.

Now suppose that you have a method declared to return a Number:

public Number returnANumber() {

...

}

The returnANumber method can return an ImaginaryNumber but not an Object. ImaginaryNumber is a Number because it's a subclass of Number. However, an Object is not necessarily a Number — it could be a String or another type.

You can override a method and define it to return a subclass of the original method, like this:

public ImaginaryNumber returnANumber() {

...

}

This technique, called covariant return type, means that the return type is allowed to vary in the same direction as the subclass.

Note: You also can use interface names as return types. In this case, the object returned must implement the specified interface.

Source: ORACLE

Java Tutorial 19: Object-Creating Objects

2012-04-26T16:25:00.001+01:00

A typical Java program creates many objects, which as you know, interact by invoking methods. Through these object interactions, a program can carry out various tasks, such as implementing a GUI, running an animation, or sending and receiving information over a network. Once an object has completed the work for which it was created, its resources are recycled for use by other objects.

Here's a small program, called CreateObjectDemo, that creates three objects: one Point object and two Rectangle objects. You will need all three source files to compile this program. Don’t worry if you cannot understand everything, we will come back later. Now you just need to copy the code and run the program.

To do this, you will need to create a new project in Eclipse then create 3 classes under the new Java project and name them Point, Rectangle and CreateObjectDemo.

In class Point, type the code below:

public class Point {

public int x = 0;

public int y = 0;

// a constructor!

public Point(int a, int b) {

x = a;

y = b;

}

In class Rectangle, type the code below:

public class Rectangle {

    public int width = 0;

    public int height = 0;

    public Point origin;

     // four constructors

    public Rectangle() {

                    origin = new Point(0, 0);

    public Rectangle(Point p) {

                    origin = p;

    public Rectangle(int w, int h) {

                    origin = new Point(0, 0);

                    width = w;

                    height = h;

    public Rectangle(Point p, int w, int h) {

                    origin = p;

                    width = w;

                    height = h;

     // a method for moving the rectangle

    public void move(int x, int y) {

                    origin.x = x;

                    origin.y = y;

     // a method for computing the area of the rectangle

    public int getArea() {

                    return width * height;

In class CreateObjectDemo, type the code below:

public class CreateObjectDemo {

public static void main(String[] args) {

// Declare and create a point object and two rectangle objects.

Point originOne = new Point(23, 94);

Rectangle rectOne = new

Rectangle(originOne, 100, 200);

Rectangle rectTwo =

new Rectangle(50, 100);

// display rectOne's width, height, and area

System.out.println("Width of rectOne: "

+ rectOne.width);

System.out.println("Height of rectOne: "

+ rectOne.height);

System.out.println("Area of rectOne: "

+ rectOne.getArea());

// set rectTwo's position

rectTwo.origin = originOne;

// display rectTwo's position

System.out.println("X Position of rectTwo: "

+ rectTwo.origin.x);

System.out.println("Y Position of rectTwo: "

+ rectTwo.origin.y);

// move rectTwo and display its new position

rectTwo.move(40, 72);

System.out.println("X Position of rectTwo: "

+ rectTwo.origin.x);

System.out.println("Y Position of rectTwo: "

+ rectTwo.origin.y);

}

This program creates, manipulates, and displays information about various objects. Here's the output:

Width of rectOne: 100

Height of rectOne: 200

Area of rectOne: 20000

X Position of rectTwo: 23

Y Position of rectTwo: 94

X Position of rectTwo: 40

Y Position of rectTwo: 72

The following three sections use the above example to describe the life cycle of an object within a program. From them, you will learn how to write code that creates and uses objects in your own programs. You will also learn how the system cleans up after an object when its life has ended.

Creating Objects

As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from theCreateObjectDemo program creates an object and assigns it to a variable:

Point originOne = new Point(23, 94);

Rectangle rectOne = new Rectangle(originOne, 100, 200);

Rectangle rectTwo = new Rectangle(50, 100);

The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.

Each of these statements has three parts:

1. Declaration: The code underlined are all variable declarations that associate a variable name with an object type.

2. Instantiation: The new keyword is a Java operator that creates the object.

3. Initialization: The new operator is followed by a call to a constructor, which initializes the new object.

Declaring a Variable to Refer to an Object

Previously, you learned that to declare a variable, you write:

type name;

This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.

You can also declare a reference variable on its own line. For example:

Point originOne;

If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.

A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne, plus a reference pointing to nothing):

Instantiating a Class

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.

Note: The phrase "instantiating a class" means the same thing as "creating an object." When you create an object, you are creating an "instance" of a class, therefore "instantiating" a class.

The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.

The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:

Point originOne = new Point(23, 94);

The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:

int height = new Rectangle().height;

This statement will be discussed in the next section.

Initializing an Object

Here's the code for the Point class:

public class Point {

public int x = 0;

public int y = 0;

//constructor

public Point(int a, int b) {

x = a;

y = b;

}

This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:

Point originOne = new Point(23, 94);

The result of executing this statement can be illustrated in the next figure:

Here's the code for the Rectangle class, which contains four constructors:

public class Rectangle {

public int width = 0;

public int height = 0;

public Point origin;

// four constructors

public Rectangle() {

origin = new Point(0, 0);

}

public Rectangle(Point p) {

origin = p;

}

public Rectangle(int w, int h) {

origin = new Point(0, 0);

width = w;

height = h;

}

public Rectangle(Point p, int w, int h) {

origin = p;

width = w;

height = h;

}

// a method for moving the rectangle

public void move(int x, int y) {

origin.x = x;

origin.y = y;

}

// a method for computing the area

// of the rectangle

public int getArea() {

return width * height;

}

Each constructor lets you provide initial values for the rectangle's size and width, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

Rectangle rectOne = new Rectangle(originOne, 100, 200);

This calls one of Rectangle's constructors that initializes origin to originOne. Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object—an object can have multiple references to it, as shown in the next figure:

The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for widthand height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:

Rectangle rectTwo = new Rectangle(50, 100);

The Rectangle constructor used in the following statement doesn't take any arguments, so it's called a no-argument constructor:

Rectangle rect = new Rectangle();

All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or the Objectconstructor if the class has no other parent. If the parent has no constructor (Object does have one), the compiler will reject the program.

Understanding constructors-How constructors differ from methods

2012-04-22T15:23:00.000+01:00

Saying that a constructor is a method is like saying that the Australian platypus is just another mammal. To understand the platypus, it is important to know how it is different from other mammals. To understand the constructor, it is similarly important to understand how it differs from a method. Any student of Java, especially one studying for certification, needs to know those differences; in this article, I will concretely spell them out. Table 1, at the end of this article, summarizes the key constructor/method distinctions.

Purpose and function

Constructors have one purpose in life: to create an instance of a class. This can also be called creating an object, as in:

Platypus p1 = new Platypus();

The purpose of methods, by contrast, is much more general. A method's basic function is to execute Java code.

Signature differences

Constructors and methods differ in three aspects of the signature, modifiers, return type, and name. Like methods, constructors can have any of the access modifiers: public, protected, private, or none (often called package or friendly). Unlike methods, constructors can take only access modifiers. Therefore, constructors cannot be abstract, final, native, static, or synchronized.

The return types are very different too. Methods can have any valid return type, or no return type, in which case the return type is given as void. Constructors have no return type, not even void.

Finally, in terms of the signature, methods and constructors have different names. Constructors have the same name as their class; by convention, methods use names other than the class name. If the Java program follows normal conventions, methods will start with a lowercase letter, constructors with an uppercase letter. Also, constructor names are usually nouns because class names are usually nouns; method names usually indicate actions.

The use of "this"

Constructors and methods use the keyword this quite differently. A method uses this to refer to the instance of the class that is executing the method. Static methods do not use this; they do not belong to a class instance, so this would have nothing to reference. Static methods belong to the class as a whole, rather than to an instance. Constructors use this to refer to another constructor in the same class with a different parameter list. Study the following code:

public class Platypus {
       String name;
       Platypus(String input) {
               name = input;
       }
       Platypus() {
               this("John/Mary Doe");
       }
       public static void main(String args[]) {
               Platypus p1 = new Platypus("digger");
               Platypus p2 = new Platypus();
       }
}

In the code, there are two constructors. The first takes a String input to name the instance. The second, taking no parameters, calls the first constructor by the default name "John/Mary Doe".

If a constructor uses this, it must be in the constructor's first line; ignoring this rule will cause the compiler to object.

The use of "super"

Methods and constructors both use super to refer to a superclass, but in different ways. Methods use super to execute an overridden method in the superclass, as the following example illustrates: age 2 of 2

class Mammal {
       void getBirthInfo() {
               System.out.println("born alive.");
       }
}
class Platypus extends Mammal {
       void getBirthInfo() {
               System.out.println("hatch from eggs");
               System.out.print("a mammal normally is ");
               super.getBirthInfo();
       }
}

In the above program, the call to super.getBirthInfo() calls the overridden method of the Mammal superclass.

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. An example follows:

public class SuperClassDemo {
        SuperClassDemo() {}
}
class Child extends SuperClassDemo {
       Child() {
               super();
       }
}

In the above (and trivial!) example, the constructor Child() includes a call to super, which causes the class SuperClassDemo to be instantiated, in addition to the Child class.

Compiler-supplied code

The new Java programmer may stumble when the compiler automatically supplies code for constructors. This happens if you write a class with no constructors; the compiler will automatically supply a no-argument constructor for you. Thus, if you write:

public class Example {}

it is functionally equivalent to writing:

public class Example {
       Example() {}
}

The compiler also automatically supplies code when you do not use super (using zero or more parameters) as the first line of a constructor. In this case, the computer automatically inserts super. Thus, if you write:

public class TestConstructors {
       TestConstructors() {}
}

it is functionally equivalent to writing:

public class TestConstructors {
       TestConstructors() {
               super;
       }
}

The sharp-eyed beginner may wonder how the above program can call the parent class's constructor when TestConstructor is not extending any class. The answer is that Java extends the Object class when you do not explicitly extend a class. The compiler automatically supplies a no-argument constructor if no constructor is explicitly declared, and automatically supplies a no-argument super call when a constructor has no explicit call to super. So the following two code snippets are functionally equivalent:

public class Example {}

and

public class Example {
       Example() {
             super;
       }
}

Inheritance

What is wrong with the following scenario? A lawyer is reading the will of A. Class. Members of the Class family are gathered around a large conference table, some sobbing gently. The lawyer reads, "I, A. Class, being of sound mind and body, leave all my constructors to my children."

The problem is that constructors cannot be inherited. Fortunately for the Class children, they will automatically inherit any of their parents' methods, so the Class children will not become totally destitute.

Remember, Java methods are inherited, constructors are not. Consider the following class:

public class Example {
        public void sayHi {
                system.out.println("Hi");
        }
        Example() {}
}
public class SubClass extends Example {
}

The SubClass class automatically inherits the sayHi method found in the parent class. However, the constructor Example() is not inherited by the SubClass.

Summarizing the differences

Just as the platypus differs from the typical mammal, so too do constructors differ from methods; specifically in their purpose, signature, and use of this and super. Additionally, constructors differ with respect to inheritance and compiler-supplied code. Keeping all these details straight can be a chore; the following table provides a convenient summary of the salient points.

Differences Between Constructors and Methods

Source: Javaworld

Java Tutorial 18: Classes: Passing Information to a Method or a Constructor

2012-04-22T15:15:00.003+01:00

The declaration for a method or a constructor declares the number and the type of the arguments for that method or constructor. For example, the following is a method that computes the monthly payments for a home loan, based on the amount of the loan, the interest rate, the length of the loan (the number of periods), and the future value of the loan:

public double computePayment(

double loanAmt,

double rate,

double futureValue,

int numPeriods) {

double interest = rate / 100.0;

double partial1 = Math.pow((1 + interest),

- numPeriods);

double denominator = (1 - partial1) / interest;

double answer = (-loanAmt / denominator)

- ((futureValue * partial1) / denominator);

return answer;

}

This method has four parameters: the loan amount, the interest rate, the future value and the number of periods. The first three are double-precision floating point numbers, and the fourth is an integer. The parameters are used in the method body and at runtime will take on the values of the arguments that are passed in.

Note: Parameters refers to the list of variables in a method declaration. Arguments are the actual values that are passed in when the method is invoked. When you invoke a method, the arguments used must match the declaration's parameters in type and order.

Parameter Types

You can use any data type for a parameter of a method or a constructor. This includes primitive data types, such as doubles, floats, and integers, as you saw in the computePayment method, and reference data types, such as objects and arrays.

Here's an example of a method that accepts an array as an argument. In this example, the method creates a new Polygon object and initializes it from an array of Point objects (assume that Point is a class that represents an x, y coordinate):

public Polygon polygonFrom(Point[] corners) {

// method body goes here

}

Note: The Java programming language doesn't let you pass methods into methods. But you can pass an object into a method and then invoke the object's methods.

Arbitrary Number of Arguments

You can use a construct called varargs to pass an arbitrary number of values to a method. You use varargs when you don't know how many of a particular type of argument will be passed to the method. It's a shortcut to creating an array manually (the previous method could have used varargs rather than an array).

To use varargs, you follow the type of the last parameter by an ellipsis (three dots, ...), then a space, and the parameter name. The method can then be called with any number of that parameter, including none.

public Polygon polygonFrom(Point... corners) {

int numberOfSides = corners.length;

double squareOfSide1, lengthOfSide1;

squareOfSide1 = (corners[1].x - corners[0].x)

* (corners[1].x - corners[0].x)

+ (corners[1].y - corners[0].y)

* (corners[1].y - corners[0].y);

lengthOfSide1 = Math.sqrt(squareOfSide1);

// more method body code follows that creates and returns a

// polygon connecting the Points

}

You can see that, inside the method, corners is treated like an array. The method can be called either with an array or with a sequence of arguments. The code in the method body will treat the parameter as an array in either case.

You will most commonly see varargs with the printing methods; for example, this printf method:

public PrintStream printf(String format, Object... args)

allows you to print an arbitrary number of objects. It can be called like this:

System.out.printf("%s: %d, %s%n", name, idnum, address);

or like this

System.out.printf("%s: %d, %s, %s, %s%n", name, idnum, address, phone, email);

or with yet a different number of arguments.

Parameter Names

When you declare a parameter to a method or a constructor, you provide a name for that parameter. This name is used within the method body to refer to the passed-in argument.

The name of a parameter must be unique in its scope. It cannot be the same as the name of another parameter for the same method or constructor, and it cannot be the name of a local variable within the method or constructor.

A parameter can have the same name as one of the class's fields. If this is the case, the parameter is said to shadow the field. Shadowing fields can make your code difficult to read and is conventionally used only within constructors and methods that set a particular field. For example, consider the following Circle class and its setOrigin method:

public class Circle {

private int x, y, radius;

public void setOrigin(int x, int y) {

...

}

The Circle class has three fields: x, y, and radius. The setOrigin method has two parameters, each of which has the same name as one of the fields. Each method parameter shadows the field that shares its name. So using the simple names x or y within the body of the method refers to the parameter, not to the field. To access the field, you must use a qualified name. This will be discussed later in this lesson in the section titled "Using the this Keyword."

Passing Primitive Data Type Arguments

Primitive arguments, such as an int or a double, are passed into methods by value. This means that any changes to the values of the parameters exist only within the scope of the method. When the method returns, the parameters are gone and any changes to them are lost. Here is an example:

public class PassPrimitiveByValue {

public static void main(String[] args) {

int x = 3;

// invoke passMethod() with

// x as argument

passMethod(x);

// print x to see if its

// value has changed

System.out.println("After invoking passMethod, x = " + x);

}

// change parameter in passMethod()

public static void passMethod(int p) {

p = 10;

}

When you run this program, the output is:

After invoking passMethod, x = 3

Passing Reference Data Type Arguments

Reference data type parameters, such as objects, are also passed into methods by value. This means that when the method returns, the passed-in reference still references the same object as before. However, the values of the object's fields can be changed in the method, if they have the proper access level.

For example, consider a method in an arbitrary class that moves Circle objects:

public void moveCircle(Circle circle, int deltaX, int deltaY) {

// code to move origin of

// circle to x+deltaX, y+deltaY

circle.setX(circle.getX() + deltaX);

circle.setY(circle.getY() + deltaY);

// code to assign a new

// reference to circle

circle = new Circle(0, 0);

}

Let the method be invoked with these arguments:

moveCircle(myCircle, 23, 56)

Inside the method, circle initially refers to myCircle. The method changes the x and y coordinates of the object that circle references (i.e., myCircle) by 23 and 56, respectively. These changes will persist when the method returns. Then circle is assigned a reference to a new Circle object with x = y = 0. This reassignment has no permanence, however, because the reference was passed in by value and cannot change. Within the method, the object pointed to by circle has changed, but, when the method returns, myCircle still references the same Circle object as before the method was called.

Java Tutorial 17: Classes-Defining Methods and Providing Constructors for Your Classes

2012-04-22T14:45:00.002+01:00

Defining Methods

Here is an example of a typical method declaration:

public double calculateAnswer(double wingSpan, int numberOfEngines,

double length, double grossTons) {

//do the calculation here

}

The only required elements of a method declaration are the method's return type, name, a pair of parentheses, (), and a body between braces, {}.

More generally, method declarations have six components, in order:

Modifiers—such as public, private.
The return type—the data type of the value returned by the method, or void if the method does not return a value.
The method name—the rules for field names apply to method names as well, but the convention is a little different.
The parameter list in parenthesis—a comma-delimited list of input parameters, preceded by their data types, enclosed by parentheses, (). If there are no parameters, you must use empty parentheses.
An exception list—to be discussed later.
The method body, enclosed between braces—the method's code, including the declaration of local variables, goes here.

Modifiers, return types, and parameters will be discussed later in this lesson. Exceptions are discussed in a later lesson.

Definition: Two of the components of a method declaration comprise the method signature—the method's name and the parameter types.

The signature of the method declared above is:

calculateAnswer(double, int, double, double)

Naming a Method

Although a method name can be any legal identifier, code conventions restrict method names. By convention, method names should be a verb in lowercase or a multi-word name that begins with a verb in lowercase, followed by adjectives, nouns, etc. In multi-word names, the first letter of each of the second and following words should be capitalized. Here are some examples:

run

runFast

getBackground

getFinalData

compareTo

setX

isEmpty

Typically, a method has a unique name within its class. However, a method might have the same name as other methods due to method overloading which is not encouraged.

Providing Constructors for Your Classes

A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type. For example, Bicycle has one constructor:

public Bicycle(int startCadence, int startSpeed, int startGear) {

gear = startGear;

cadence = startCadence;

speed = startSpeed;

}

To create a new Bicycle object called myBike, a constructor is called by the new operator:

Bicycle myBike = new Bicycle(30, 0, 8);

new Bicycle(30, 0, 8) creates space in memory for the object and initializes its fields.

Although Bicycle only has one constructor, it could have others, including a no-argument constructor:

public Bicycle() {

gear = 1;

cadence = 10;

speed = 0;

}

Bicycle yourBike = new Bicycle(); invokes the no-argument constructor to create a new Bicycle object called yourBike.

Both constructors could have been declared in Bicycle because they have different argument lists. As with methods, the Java platform differentiates constructors on the basis of the number of arguments in the list and their types. You cannot write two constructors that have the same number and type of arguments for the same class, because the platform would not be able to tell them apart. Doing so causes a compile-time error.

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.

You can use a superclass constructor yourself. The MountainBike class at the beginning of this lesson did just that. This will be discussed later, in the lesson on interfaces and inheritance.

You can use access modifiers in a constructor's declaration to control which other classes can call the constructor.

Note: If another class cannot call a MyClass constructor, it cannot directly create MyClass objects.

Java Tutorial 16: Classes-Declaring Classes and Declaring Member Variables

2012-04-22T14:08:00.000+01:00

The introduction to object-oriented Programming Concepts used a bicycle class as an example, with racing bikes, mountain bikes, and tandem bikes as subclasses. Here is sample code for a possible implementation of a Bicycle class, to give you an overview of a class declaration. Subsequent sections of this lesson will back up and explain class declarations step by step. For the moment, don't concern yourself with the details.

public class Bicycle {

// the Bicycle class has three fields

public int cadence;

public int gear;

public int speed;

// the Bicycle class has one constructor

public Bicycle(int startCadence, int startSpeed, int startGear) {

gear = startGear;

cadence = startCadence;

speed = startSpeed;

}

// the Bicycle class has four methods

public void setCadence(int newValue) {

cadence = newValue;

}

public void setGear(int newValue) {

gear = newValue;

}

public void applyBrake(int decrement) {

speed -= decrement;

}

public void speedUp(int increment) {

speed += increment;

}

A class declaration for a MountainBike class that is a subclass of Bicycle might look like this:

public class MountainBike extends Bicycle {

// the MountainBike subclass has one field

public int seatHeight;

// the MountainBike subclass has one constructor

public MountainBike(int startHeight, int startCadence,

int startSpeed, int startGear) {

super(startCadence, startSpeed, startGear);

seatHeight = startHeight;

}

// the MountainBike subclass has one method

public void setHeight(int newValue) {

seatHeight = newValue;

}

MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it (mountain bikes have seats that can be moved up and down as the terrain demands).

Declaring Classes

You've seen classes defined in the following way:

class MyClass {

// field, constructor, and method declarations

}

This is a class declaration. The class body (the area between the braces) contains all the code that provides for the life cycle of the objects created from the class: constructors for initializing new objects, declarations for the fields that provide the state of the class and its objects, and methods to implement the behavior of the class and its objects.

The preceding class declaration is a minimal one. It contains only those components of a class declaration that are required. You can provide more information about the class, such as the name of its superclass, whether it implements any interfaces, and so on, at the start of the class declaration. For example,

class MyClass extends MySuperClass implements YourInterface {

// field, constructor, and

// method declarations

}

means that MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.

You can also add modifiers like public or private at the very beginning—so you can see that the opening line of a class declaration can become quite complicated. The modifiers public and private, which determine what other classes can access MyClass, are discussed later . The lesson on interfaces and inheritance will explain how and why you would use the extends and implements keywords in a class declaration. For the moment you do not need to worry about these extra complications.

In general, class declarations can include these components, in order:

Modifiers such as public, private, and a number of others that you will encounter later.
The class name, with the initial letter capitalized by convention.
The name of the class's parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface.
The class body, surrounded by braces, {}.

Declaring Member Variables

There are several kinds of variables:

Member variables in a class—these are called fields.
Variables in a method or block of code—these are called local variables.
Variables in method declarations—these are called parameters.

The Bicycle class uses the following lines of code to define its fields:

public int cadence;

public int gear;

public int speed;

Field declarations are composed of three components, in order:

Zero or more modifiers, such as public or private.

The field's type.

The field's name.

The fields of Bicycle are named cadence, gear, and speed and are all of data type integer (int). The public keyword identifies these fields as public members, accessible by any object that can access the class.

Access Modifiers

The first (left-most) modifier used lets you control what other classes have access to a member field. For the moment, consider only public and private.

public modifier—the field is accessible from all classes.

private modifier—the field is accessible only within its own class.

In the spirit of encapsulation, it is common to make fields private. This means that they can only be directly accessed from the Bicycle class.

Types

All variables must have a type. You can use primitive types such as int, float, boolean, etc. Or you can use reference types, such as strings, arrays, or objects.

Variable Names

All variables, whether they are fields, local variables, or parameters, follow the same naming rules and conventions that were covered in the Language Basics lesson, Variables—Naming.

In this lesson, be aware that the same naming rules and conventions are used for method and class names, except that

the first letter of a class name should be capitalized, and

the first (or only) word in a method name should be a verb.

Java Tutorial 15: Control Flow Statements- Branching Statements

2012-04-22T10:56:00.000+01:00

The break Statement

The break statement has two forms: labelled and unlabelled. You saw the unlabelled form in the previous discussion of the switch statement. You can also use an unlabelled break to terminate a for, while, or do-while loop, as shown in the following Break Demo program:

class BreakDemo {

public static void main(String[] args) {

int[] arrayOfInts =

{ 32, 87, 3, 589,

12, 1076, 2000,

8, 622, 127 };

int searchfor = 12;

int i;

boolean foundIt = false;

for (i = 0; i < arrayOfInts.length; i++) {

if (arrayOfInts[i] == searchfor) {

foundIt = true;

break;

}

if (foundIt) {

System.out.println("Found " + searchfor + " at index " + i);

} else {

System.out.println(searchfor + " not in the array");

}

This program searches for the number 12 in an array. The break statement, shown in boldface, terminates the for loop when that value is found. Control flow then transfers to the statement after the for loop. This program's output is:

Found 12 at index 4

An unlabelled break statement terminates the inner most switch, for, while, or do-while statement, but a labelled break terminates an outer statement. The following program, BreakWithLabelDemo, is similar to the previous program, but uses nested for loops to search for a value in a two-dimensional array. When the value is found, a labelled break terminates the outer for loop (labelled "search"):

class BreakWithLabelDemo {

public static void main(String[] args) {

int[][] arrayOfInts = {

{ 32, 87, 3, 589 },

{ 12, 1076, 2000, 8 },

{ 622, 127, 77, 955 }

};

int searchfor = 12;

int i;

int j = 0;

boolean foundIt = false;

search:

for (i = 0; i < arrayOfInts.length; i++) {

for (j = 0; j < arrayOfInts[i].length;

j++) {

if (arrayOfInts[i][j] == searchfor) {

foundIt = true;

break search;

}

if (foundIt) {

System.out.println("Found " + searchfor +

" at " + i + ", " + j);

} else {

System.out.println(searchfor +

" not in the array");

}

This is the output of the program.

Found 12 at 1, 0

The break statement terminates the labelled statement; it does not transfer the flow of control to the label. Control flow is transferred to the statement immediately following the labelled (terminated) statement.

The continue Statement

The continue statement skips the current iteration of a for, while , or do-while loop. The unlabelled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop. The following program, ContinueDemo , steps through a String, counting the occurrences of the letter "p". If the current character is not a p, the continue statement skips the rest of the loop and proceeds to the next character. If it is a "p", the program increments the letter count.

class ContinueDemo {

public static void main(String[] args) {

String searchMe

= "peter piper picked a " +

"peck of pickled peppers";

int numPs = 0;

for (int i = 0; i < searchMe.length(); i++) {

// interested only in p's

if (searchMe.charAt(i) != 'p')

continue;

// process p's

numPs++;

}

System.out.println("Found " +

numPs + " p's in the string.");

}

Here is the output of this program:

Found 9 p's in the string.

To see this effect more clearly, try removing the continue statement and recompiling. When you run the program again, the count will be wrong, saying that it found 35 p's instead of 9.

A labelled continue statement skips the current iteration of an outer loop marked with the given label. The following example program, ContinueWithLabelDemo, uses nested loops to search for a substring within another string. Two nested loops are required: one to iterate over the substring and one to iterate over the string being searched. The following program, ContinueWithLabelDemo, uses the labelled form of continue to skip an iteration in the outer loop.

class ContinueWithLabelDemo {

public static void main(String[] args) {

String searchMe

= "Look for a substring in me";

String substring = "sub";

boolean foundIt = false;

int max = searchMe.length() -

substring.length();

test:

for (int i = 0; i <= max; i++) {

int n = substring.length();

int j = i;

int k = 0;

while (n-- != 0) {

if (searchMe.charAt(j++)

!= substring.charAt(k++)) {

continue test;

}

foundIt = true;

break test;

}

System.out.println(foundIt ?

"Found it" : "Didn't find it");

}

Here is the output from this program.

Found it

The return Statement

The last of the branching statements is the return statement. The return statement exits from the current method, and control flow returns to where the method was invoked. The return statement has two forms: one that returns a value, and one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return keyword.

return ++count;

The data type of the returned value must match the type of the method's declared return value. When a method is declared void, use the form of return that doesn't return a value.

return;

The Classes and Objects lesson will cover everything you need to know about writing methods.

Summary of Control Flow Statements

The if-then statement is the most basic of all the control flow statements. It tells your program to execute a certain section of code only if a particular test evaluates to true. The if-then-else statement provides a secondary path of execution when an "if" clause evaluates to false. Unlike if-then and if-then-else, the switch statement allows for any number of possible execution paths. The while and do-while statements continually execute a block of statements while a particular condition is true. The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once. The for statement provides a compact way to iterate over a range of values. It has two forms, one of which was designed for looping through collections and arrays.

Java Tutorial 14: Control Flow Statements- The for Statement

2012-04-19T17:02:00.003+01:00

The for statement provides a compact way to iterate over a range of values. Programmers often refer to it as the "for loop" because of the way in which it repeatedly loops until a particular condition is satisfied. The general form of the for statement can be expressed as follows:

for (initialization; termination;

increment) {

statement(s)

}

When using this version of the for statement, keep in mind that:

The initialization expression initializes the loop; it's executed once, as the loop begins. When the termination expression evaluates to false, the loop terminates. The increment expression is invoked after each iteration through the loop; it is perfectly acceptable for this expression to increment or decrement a value.

The following program, ForDemo, uses the general form of the for statement to print the numbers 1 through 10 to standard output:

class ForDemo {

public static void main(String[] args){

for(int i=1; i<11; i++){

System.out.println("Count is: "

+ i);

}

The output of this program is:

Count is: 1

Count is: 2

Count is: 3

Count is: 4

Count is: 5

Count is: 6

Count is: 7

Count is: 8

Count is: 9

Count is: 10

Notice how the code declares a variable within the initialization expression. The scope of this variable extends from its declaration to the end of the block governed by the for statement, so it can be used in the termination and increment expressions as well. If the variable that controls a for statement is not needed outside of the loop, it's best to declare the variable in the initialization expression. The names i, j, and k are often used to control for loops; declaring them within the initialization expression limits their life span and reduces errors.

The three expressions of the for loop are optional; an infinite loop can be created as follows:

// infinite loop

for ( ; ; ) {

// your code goes here

}

The for statement also has another form designed for iteration through Collections and arrays This form is sometimes referred to as the enhanced for statement, and can be used to make your loops more compact and easy to read. To demonstrate, consider the following array, which holds the numbers 1 through 10:

int[] numbers = {1,2,3,4,5,6,7,8,9,10};

The following program, EnhancedForDemo, uses the enhanced for to loop through the array:

class EnhancedForDemo {

public static void main(String[] args){

int[] numbers =

{1,2,3,4,5,6,7,8,9,10};

for (int item : numbers) {

System.out.println("Count is: "

+ item);

}

In this example, the variable item holds the current value from the numbers array. The output from this program is the same as before:

Count is: 1

Count is: 2

Count is: 3

Count is: 4

Count is: 5

Count is: 6

Count is: 7

Count is: 8

Count is: 9

Count is: 10

We recommend using this form of the for statement instead of the general form whenever possible.

Java Tutorial 13: Control Flow Statements- The while and do-while Statements

2012-04-19T16:53:00.002+01:00

The while statement continually executes a block of statements while a particular condition is true. Its syntax can be expressed as:

while (expression) {

statement(s)

}

The while statement evaluates expression, which must return a boolean value. If the expression evaluates to true, the while statement executes the statement(s) in the while block. The while statement continues testing the expression and executing its block until the expression evaluates to false. Using the while statement to print the values from 1 through 10 can be accomplished as in the following WhileDemo program:

class WhileDemo {

public static void main(String[] args){

int count = 1;

while (count < 11) {

System.out.println("Count is: "

+ count);

count++;

}

You can implement an infinite loop using the while statement as follows:

while (true){

// your code goes here

}

The Java programming language also provides a do-while statement, which can be expressed as follows:

do {

statement(s)

} while (expression);

The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once, as shown in the following DoWhileDemo program:

class DoWhileDemo {

public static void main(String[] args){

int count = 1;

do {

System.out.println("Count is: "

+ count);

count++;

} while (count < 11);

}

How to use these tutorials

2012-04-19T14:21:00.003+01:00

Just to answer some peoples email and give you a few tips on how to use these tutorials. Most of these tutorials that I have posted are from ORACLE website. By the end of the day, these are the most official material.

I have deleted a few bits here and there to make it easier to follow. These things either are not very important for beginners or not important at all. You can always come back to them when you are “fluent” with the language. I have also changed some of the codes to make it easier, again, to make it easy to follow.

To use these tutorial, I recommend to follow the below steps

Read the tutorials, including the code, and try to understand everything.
Read the tutorials a few more times and try to memorize all the important concept, definition, etc.
TYPR (do not copy) the code into Eclipse and run the application, change some variables and see what will happen. Don’t worry; you cannot break your computer.
Now close the tutorial, see if you can remember the code.
In later tutorial, if you forget something, don’t just ignore them. Go back and read them again.

Hope this helps!

Java Tutorial 12: Control Flow Statements- The switch Statement

2012-04-18T16:28:00.001+01:00

Unlike if-then and if-then-else statements, the switch statement can have a number of possible execution paths. A switch works with the byte, short, char, and int primitive data types. It also works with enumerated types (discussed later), the String class, and a few special classes that wrap certain primitive types: Character, Byte, Short, and Integer (discussed in Numbers and Strings).

The following code example, SwitchDemo, declares an int named month whose value represents a month. The code displays the name of the month, based on the value of month, using the switch statement.

public class SwitchDemo {

public static void main(String[] args) {

int month = 8;

String monthString;

switch (month) {

case 1: monthString = "January";

break;

case 2: monthString = "February";

break;

case 3: monthString = "March";

break;

case 4: monthString = "April";

break;

case 5: monthString = "May";

break;

case 6: monthString = "June";

break;

case 7: monthString = "July";

break;

case 8: monthString = "August";

break;

case 9: monthString = "September";

break;

case 10: monthString = "October";

break;

case 11: monthString = "November";

break;

case 12: monthString = "December";

break;

default: monthString = "Invalid month";

break;

}

System.out.println(monthString);

}

In this case, August is printed to standard output.

The body of a switch statement is known as a switch block. A statement in the switch block can be labelled with one or more case or default labels. The switch statement evaluates its expression, then executes all statements that follow the matching case label.

You could also display the name of the month with if-then-else statements:

int month = 8;

if (month == 1) {

System.out.println("January");

} else if (month == 2) {

System.out.println("February");

}

... // and so on

Deciding whether to use if-then-else statements or a switch statement is based on readability and the expression that the statement is testing. An if-then-else statement can test expressions based on ranges of values or conditions, whereas a switch statement tests expressions based only on a single integer, enumerated value, or String object.

Another point of interest is the break statement. Each break statement terminates the enclosing switch statement. Control flow continues with the first statement following the switch block. The break statements are necessary because without them, statements in switch blocks fall through: All statements after the matching case label are executed in sequence, regardless of the expression of subsequent case labels, until a break statement is encountered. The program witchDemoFallThrough shows statements in a switch block that fall through. The program displays the month corresponding to the integer month and the months that follow in the year: (This demo include ArrayList and for loop that we will come back to that later.)

public class SwitchDemoFallThrough {

public static void main(String args[]) {

java.util.ArrayList<String> futureMonths =

new java.util.ArrayList<String>();

int month = 8;

switch (month) {

case 1: futureMonths.add("January");

case 2: futureMonths.add("February");

case 3: futureMonths.add("March");

case 4: futureMonths.add("April");

case 5: futureMonths.add("May");

case 6: futureMonths.add("June");

case 7: futureMonths.add("July");

case 8: futureMonths.add("August");

case 9: futureMonths.add("September");

case 10: futureMonths.add("October");

case 11: futureMonths.add("November");

case 12: futureMonths.add("December");

break;

default: break;

}

if (futureMonths.isEmpty()) {

System.out.println("Invalid month number");

} else {
// the code below utilize the enhanced for statement, which will be discussed in tutorial 14.

for (String monthName : futureMonths) {

System.out.println(monthName);

}

This is the output from the code:

August

September

October

November

December

Technically, the final break is not required because flow falls out of the switch statement. Using a break is recommended so that modifying the code is easier and less error prone. The default section handles all values that are not explicitly handled by one of the case sections.

The following code example, SwitchDemo2, shows how a statement can have multiple case labels. The code example calculates the number of days in a particular month:

class SwitchDemo2 {

public static void main(String[] args) {

int month = 2;

int year = 2000;

int numDays = 0;

switch (month) {

case 1: case 3: case 5:

case 7: case 8: case 10:

case 12:

numDays = 31;

break;

case 4: case 6:

case 9: case 11:

numDays = 30;

break;

case 2:

if (year % 4 == 0)

{numDays = 29;}

else

{numDays = 28;}

break;

default:

System.out.println("Invalid month.");

break;

}

System.out.println("Number of Days = "

+ numDays);

}

This is the output from the code:

Number of Days = 29

Using Strings in switch Statements

In Java SE 7 and later, you can use a String object in the switch statement's expression. The following code example, StringSwitchDemo, displays the number of the month based on the value of the String named month:

public class StringSwitchDemo {

public static int getMonthNumber(String month) {

int monthNumber = 0;

if (month == null) {

return monthNumber;

}

switch (month.toLowerCase()) {

case "january":

monthNumber = 1;

break;

case "february":

monthNumber = 2;

break;

case "march":

monthNumber = 3;

break;

case "april":

monthNumber = 4;

break;

case "may":

monthNumber = 5;

break;

case "june":

monthNumber = 6;

break;

case "july":

monthNumber = 7;

break;

case "august":

monthNumber = 8;

break;

case "september":

monthNumber = 9;

break;

case "october":

monthNumber = 10;

break;

case "november":

monthNumber = 11;

break;

case "december":

monthNumber = 12;

break;

default:

monthNumber = 0;

break;

}

return monthNumber;

}

public static void main(String[] args) {

String month = "August";

int returnedMonthNumber =

StringSwitchDemo.getMonthNumber(month);

if (returnedMonthNumber == 0) {

System.out.println("Invalid month");

} else {

System.out.println(returnedMonthNumber);

}

The output from this code is 8.

The String in the switch expression is compared with the expressions associated with each case label as if the String.equals method were being used. In order for the StringSwitchDemo example to accept any month regardless of case, month is converted to lowercase (with the toLowerCase method), and all the strings associated with the case labels are in lowercase.

Note: This example checks if the expression in the switch statement is null. Ensure that the expression in any switch statement is not null to prevent a NullPointerException from being thrown.

Java Tutorial 11: Control Flow Statements-The if-then and if-then-else Statements

2012-04-18T15:01:00.006+01:00

The statements inside your source files are generally executed from top to bottom, in the order that they appear. Control flow statements, however, break up the flow of execution by employing decision making, looping, and branching, enabling your program to conditionally execute particular blocks of code. This section describes the decision-making statements (if-then, if-then-else, switch), the looping statements (for, while, do-while), and the branching statements (break, continue, return) supported by the Java programming language.

The if-then Statement

The if-then statement is the most basic of all the control flow statements. It tells your program to execute a certain section of code only if a particular test evaluates to true. For example, the Bicycle class could allow the brakes to decrease the bicycle's speed only if the bicycle is already in motion. One possible implementation of the applyBrakes method could be as follows:

void applyBrakes() {

// the "if" clause: bicycle must be moving

if (isMoving){

// the "then" clause: decrease current speed

currentSpeed--;

}

If this test evaluates to false (meaning that the bicycle is not in motion), control jumps to the end of the if-then statement.

The if-then-else Statement

The if-then-else statement provides a secondary path of execution when an "if" clause evaluates to false. You could use an if-then-else statement in the applyBrakes method to take some action if the brakes are applied when the bicycle is not in motion. In this case, the action is to simply print an error message stating that the bicycle has already stopped.

void applyBrakes() {

if (isMoving) {

currentSpeed--;

} else {

System.err.println("The bicycle has " + "already stopped!");

}

The following program, IfElseDemo, assigns a grade based on the value of a test score: an A for a score of 90% or above, a B for a score of 80% or above, and so on.

class IfElseDemo {

public static void main(String[] args) {

int testscore = 76;

char grade;

if (testscore >= 90) {

grade = 'A';

} else if (testscore >= 80) {

grade = 'B';

} else if (testscore >= 70) {

grade = 'C';

} else if (testscore >= 60) {

grade = 'D';

} else {

grade = 'F';

}

System.out.println("Grade = " + grade);

}

The output from the program is:

Grade = C

You may have noticed that the value of testscore can satisfy more than one expression in the compound statement: 76 >= 70 and 76 >= 60. However, once a condition is satisfied, the appropriate statements are executed (grade = 'C';) and the remaining conditions are not evaluated.

Java Tutorial 10: Expressions, Statements, and Blocks

2012-04-18T14:52:00.005+01:00

Operators are used in building expressions, which compute values; expressions are the core components of statements; statements may be grouped into blocks.

Expressions

An expression is a construct made up of variables, operators, and method invocations, which are constructed according to the syntax of the language, which evaluates to a single value. You've already seen examples of expressions, illustrated in bold below:

int cadence = 0;

anArray[0] = 100;

System.out.println("Element 1 at index 0: " + anArray[0]);

int result = 1 + 2; // result is now 3

if (value1 == value2)

System.out.println("value1 == value2");

The data type of the value returned by an expression depends on the elements used in the expression. The expression cadence = 0 returns an int because the assignment operator returns a value of the same data type as its left-hand operand; in this case, cadence is an int. As you can see from the other expressions, an expression can return other types of values as well, such as boolean or String.

The Java programming language allows you to construct compound expressions from various smaller expressions as long as the data type required by one part of the expression matches the data type of the other. Here's an example of a compound expression:

1 * 2 * 3

In this particular example, the order in which the expression is evaluated is unimportant because the result of multiplication is independent of order; the outcome is always the same, no matter in which order you apply the multiplications. However, this is not true of all expressions. For example, the following expression gives different results, depending on whether you perform the addition or the division operation first:

x + y / 100 // ambiguous

You can specify exactly how an expression will be evaluated using balanced parenthesis: ( and ). For example, to make the previous expression unambiguous, you could write the following:

(x + y) / 100 // unambiguous, recommended

If you don't explicitly indicate the order for the operations to be performed, the order is determined by the precedence assigned to the operators in use within the expression. Operators that have a higher precedence get evaluated first. For example, the division operator has a higher precedence than does the addition operator. Therefore, the following two statements are equivalent:

x + y / 100

x + (y / 100) // unambiguous, recommended

When writing compound expressions, be explicit and indicate with parentheses which operators should be evaluated first. This practice makes code easier to read and to maintain.

Statements

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon (;).

Assignment expressions

Any use of ++ or --

Method invocations

Object creation expressions

Such statements are called expression statements. Here are some examples of expression statements.

// assignment statement

aValue = 8933.234;

// increment statement

aValue++;

// method invocation statement

System.out.println("Hello World!");

// object creation statement

Bicycle myBike = new Bicycle();

In addition to expression statements, there are two other kinds of statements: declaration statements and control flow statements. A declaration statement declares a variable. You've seen many examples of declaration statements already:

// declaration statement

double aValue = 8933.234;

Finally, control flow statements regulate the order in which statements get executed. You'll learn about control flow statements in the next section.

Blocks

A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed. The following example, BlockDemo, illustrates the use of blocks:

class BlockDemo {

public static void main(String[] args) {

boolean condition = true;

if (condition) { // begin block 1

System.out.println("Condition is true.");

} // end block one

else { // begin block 2

System.out.println("Condition is false.");

} // end block 2

}

Java Tutorial 9: Operators-Equality, Relational, and Conditional Operators

2012-04-16T20:45:00.001+01:00

The Equality and Relational Operators

The equality and relational operators determine if one operand is greater than, less than, equal to, or not equal to another operand. The majority of these operators will probably look familiar to you as well. Keep in mind that you must use "==", not "=", when testing if two primitive values are equal.

== equal to

!= not equal to

> greater than

>= greater than or equal to

< less than

<= less than or equal to

The following program, ComparisonDemo, tests the comparison operators:

class ComparisonDemo {

public static void main(String[] args){

int value1 = 1;

int value2 = 2;

if(value1 == value2)

System.out.println("value1 == value2");

if(value1 != value2)

System.out.println("value1 != value2");

if(value1 > value2)

System.out.println("value1 > value2");

if(value1 < value2)

System.out.println("value1 < value2");

if(value1 <= value2)

System.out.println("value1 <= value2");

}

Output:

value1 != value2

value1 < value2

value1 <= value2

The Conditional Operators

The && and || operators perform Conditional-AND and Conditional-OR operations on two boolean expressions. These operators exhibit "short-circuiting" behavior, which means that the second operand is evaluated only if needed.

&& Conditional-AND

|| Conditional-OR

The following program, ConditionalDemo1, tests these operators:

class ConditionalDemo1 {

public static void main(String[] args){

int value1 = 1;

int value2 = 2;

if((value1 == 1) && (value2 == 2))

System.out.println("value1 is 1 AND value2 is 2");

if((value1 == 1) || (value2 == 1))

System.out.println("value1 is 1 OR value2 is 1");

}

Another conditional operator is ?:,

which can be thought of as shorthand for an if-then-else statement (discussed in the Control Flow Statements section of this lesson). This operator is also known as the ternary operator because it uses three operands. In the following example, this operator should be read as: "If someCondition is true, assign the value of value1 to result. Otherwise, assign the value of value2 to result."

The following program, ConditionalDemo2, tests the ?: operator:

class ConditionalDemo2 {

public static void main(String[] args){

int value1 = 1;

int value2 = 2;

int result;

boolean someCondition = true;

result = someCondition ? value1 : value2;

System.out.println(result);

}

Because someCondition is true, this program prints "1" to the screen. Use the ?: operator instead of an if-then-else statement if it makes your code more readable; for example, when the expressions are compact and without side-effects (such as assignments).

The Type Comparison Operator instanceof

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

The following program, InstanceofDemo, defines a parent class (named Parent), a simple interface (named MyInterface), and a child class (named Child) that inherits from the parent and implements the interface. (Don't worry if you can't understand everything, we'll comeback to this later).

class InstanceofDemo {

public static void main(String[] args) {

Parent obj1 = new Parent();

Parent obj2 = new Child();

System.out.println("obj1 instanceof Parent: "

+ (obj1 instanceof Parent));

System.out.println("obj1 instanceof Child: "

+ (obj1 instanceof Child));

System.out.println("obj1 instanceof MyInterface: "

+ (obj1 instanceof MyInterface));

System.out.println("obj2 instanceof Parent: "

+ (obj2 instanceof Parent));

System.out.println("obj2 instanceof Child: "

+ (obj2 instanceof Child));

System.out.println("obj2 instanceof MyInterface: "

+ (obj2 instanceof MyInterface));

}

class Parent {}

class Child extends Parent implements MyInterface {}

interface MyInterface {}

Output:

obj1 instanceof Parent: true

obj1 instanceof Child: false

obj1 instanceof MyInterface: false

obj2 instanceof Parent: true

obj2 instanceof Child: true

obj2 instanceof MyInterface: true

When using the instanceof operator, keep in mind that null is not an instance of anything.

Summary of Operators

The following quick reference summarizes the operators supported by the Java programming language.

Simple Assignment Operator

= Simple assignment operator

Arithmetic Operators

+ Additive operator (also used for String concatenation)

- Subtraction operator

* Multiplication operator

/ Division operator

% Remainder operator

Unary Operators

+ Unary plus operator; indicates positive value (numbers are positive without this, however)

- Unary minus operator; negates an expression

++ Increment operator; increments a value by 1

-- Decrement operator; decrements a value by 1

! Logical complement operator; inverts the value of a boolean

Equality and Relational Operators

== Equal to

!= Not equal to

> Greater than

>= Greater than or equal to

< Less than

<= Less than or equal to

Conditional Operators

&& Conditional-AND

|| Conditional-OR

?: Ternary (shorthand for if-then-else statement)

Type Comparison Operator

instanceof Compares an object to a specified type

Java Tutorial 8: Operators-Assignment, Arithmetic, and Unary Operators

2012-04-16T20:25:00.004+01:00

The Simple Assignment Operator

One of the most common operators that you'll encounter is the simple assignment operator "=". You saw this operator in the Bicycle class; it assigns the value on its right to the operand on its left:

int cadence = 0;

int speed = 0;

int gear = 1;

This operator can also be used on objects to assign object references, as discussed in Creating Objects.

The Arithmetic Operators

The Java programming language provides operators that perform addition, subtraction, multiplication, and division. "%" divides one operand by another and returns the remainder as its result.

+ additive operator (also used for String concatenation)

- subtraction operator

* multiplication operator

/ division operator

% remainder operator,

You can combine the arithmetic operators with the simple assignment operator to create compound assignments. For example, x+=1; and x=x+1; both increment the value of x by 1. The + operator can also be used for concatenating (joining) two strings together, as shown in the following ConcatDemo program:

The Unary Operators

The unary operators require only one operand; they perform various operations such as incrementing/decrementing a value by one, negating an expression, or inverting the value of a boolean.

+ Unary plus operator; indicates positive value (numbers are positive without this)

- Unary minus operator; negates

++ Increment operator; increments a value by 1

-- Decrement operator; decrements a value by 1

! Logical complement operator; inverts the value of a boolean

The increment/decrement operators can be applied before (prefix) or after (postfix) the operand. The code result++; and ++result; will both end in result being incremented by one. The only difference is that the prefix version (++result) evaluates to the incremented value, whereas the postfix version (result++) evaluates to the original value. If you are just performing a simple increment/decrement, it doesn't really matter which version you choose. But if you use this operator in part of a larger expression, the one that you choose may make a significant difference.

The following program, PrePostDemo, illustrates the prefix/postfix unary increment operator:

class PrePostDemo {

public static void main(String[] args){

int i = 3;

i++;

// prints 4

System.out.println(i);

++i;

// prints 5

System.out.println(i);

// prints 6

System.out.println(++i);

// prints 6

System.out.println(i++);

// prints 7

System.out.println(i);

}

Java Tutorial 8: Operators-Introduction

2012-04-16T17:13:00.001+01:00

Operators are special symbols that perform specific operations on one, two, or three operands, and then return a result.

The operators in the following table are listed according to precedence order. The closer to the top of the table an operator appears, the higher its precedence. Operators with higher precedence are evaluated before operators with relatively lower precedence. Operators on the same line have equal precedence. When operators of equal precedence appear in the same expression, a rule must govern which is evaluated first. All binary operators except for the assignment operators are evaluated from left to right; assignment operators are evaluated right to left.

In general-purpose programming, certain operators tend to appear more frequently than others; for example, the assignment operator "=" is far more common than the unsigned right shift operator ">>>". With that in mind, the following discussion focuses first on the operators that you're most likely to use on a regular basis, and ends focusing on those that are less common. Each discussion is accompanied by sample code that you can compile and run. Studying its output will help reinforce what you've just learned.

Java Tutorial 7: Variables-array

2012-04-15T17:25:00.000+01:00

An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.

Each item in an array is called an element, and each element is accessed by its numerical index. As shown in the above illustration, numbering begins with 0. The 9th element, for example, would therefore be accessed at index 8.

The following program, ArrayDemo, creates an array of integers, puts some values in it, and prints each value to standard output.

class ArrayDemo {

public static void main(String[] args) {

// declares an array of integers

int[] anArray;

// allocates memory for 10 integers

anArray = new int[10];

// initialize first element

anArray[0] = 100;

// initialize second element

anArray[1] = 200;

// etc.

anArray[2] = 300;

anArray[3] = 400;

anArray[4] = 500;

anArray[5] = 600;

anArray[6] = 700;

anArray[7] = 800;

anArray[8] = 900;

anArray[9] = 1000;

System.out.println("Element at index 0: "

+ anArray[0]);

System.out.println("Element at index 1: "

+ anArray[1]);

System.out.println("Element at index 2: "

+ anArray[2]);

System.out.println("Element at index 3: "

+ anArray[3]);

System.out.println("Element at index 4: "

+ anArray[4]);

System.out.println("Element at index 5: "

+ anArray[5]);

System.out.println("Element at index 6: "

+ anArray[6]);

System.out.println("Element at index 7: "

+ anArray[7]);

System.out.println("Element at index 8: "

+ anArray[8]);

System.out.println("Element at index 9: "

+ anArray[9]);

}

The output from this program is:

Element at index 0: 100

Element at index 1: 200

Element at index 2: 300

Element at index 3: 400

Element at index 4: 500

Element at index 5: 600

Element at index 6: 700

Element at index 7: 800

Element at index 8: 900

Element at index 9: 1000

In a real-world programming situation, you'd probably use one of the supported looping constructs to iterate through each element of the array, rather than write each line individually as shown above. However, this example clearly illustrates the array syntax.

Declaring a Variable to Refer to an Array

The above program declares anArray with the following line of code:

// declares an array of integers

int[] anArray;

Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written as type[], where type is the data type of the contained elements; the square brackets are special symbols indicating that this variable holds an array. The size of the array is not part of its type (which is why the brackets are empty). An array's name can be anything you want, provided that it follows the rules and conventions as previously discussed in the naming section. As with variables of other types, the declaration does not actually create an array — it simply tells the compiler that this variable will hold an array of the specified type.

Similarly, you can declare arrays of other types:

double[] anArrayOfDoubles;

boolean[] anArrayOfBooleans;

String[] anArrayOfStrings;

Creating, Initializing, and Accessing an Array

One way to create an array is with the new operator. The next statement in the ArrayDemo program allocates an array with enough memory for ten integer elements and assigns the array to the anArray variable.

// create an array of integers

anArray = new int[10];

If this statement were missing, the compiler would print an error like the following, and compilation would fail:

ArrayDemo.java:4: Variable anArray may not have been initialized.

The next few lines assign values to each element of the array:

anArray[0] = 100; // initialize first element

anArray[1] = 200; // initialize second element

anArray[2] = 300; // etc.

Each array element is accessed by its numerical index:

System.out.println("Element 1 at index 0: " + anArray[0]);

System.out.println("Element 2 at index 1: " + anArray[1]);

System.out.println("Element 3 at index 2: " + anArray[2]);

Alternatively, you can use the shortcut syntax to create and initialize an array:

int[] anArray = {

100, 200, 300,

400, 500, 600,

700, 800, 900, 1000

};

Here the length of the array is determined by the number of values provided between { and }.

You can also declare an array of arrays (also known as a multidimensional array) by using two or more sets of square brackets, such as String[][] names. Each element, therefore, must be accessed by a corresponding number of index values.

In the Java programming language, a multidimensional array is simply an array whose components are themselves arrays. A consequence of this is that the rows are allowed to vary in length, as shown in the following MultiDimArrayDemoprogram:

class MultiDimArrayDemo {

public static void main(String[] args) {

String[][] names = {

{"Mr. ", "Mrs. ", "Ms. "},

{"Smith", "Jones"}

};

// Mr. Smith

System.out.println(names[0][0] + names[1][0]);

// Ms. Jones

System.out.println(names[0][2] + names[1][1]);

}

The output from this program is:

Mr. Smith

Ms. Jones

Finally, you can use the built-in length property to determine the size of any array. The code

System.out.println(anArray.length);

will print the array's size to standard output.

Copying Arrays

The System class has an arraycopy method that you can use to efficiently copy data from one array into another:

public static void arraycopy(Object src, int srcPos,

Object dest, int destPos, int length)

The two Object arguments specify the array to copy from and the array to copy to. The three int arguments specify the starting position in the source array, the starting position in the destination array, and the number of array elements to copy.

The following program, ArrayCopyDemo, declares an array of char elements, spelling the word "decaffeinated". It uses arraycopy to copy a subsequence of array components into a second array:

class ArrayCopyDemo {

public static void main(String[] args) {

char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',

'i', 'n', 'a', 't', 'e', 'd' };

char[] copyTo = new char[7];

System.arraycopy(copyFrom, 2, copyTo, 0, 7);

System.out.println(new String(copyTo));

}

The output from this program is:

caffein

Summary of Variables

The Java programming language uses both "fields" and "variables" as part of its terminology. Instance variables (non-static fields) are unique to each instance of a class. Class variables (static fields) are fields declared with the static modifier; there is exactly one copy of a class variable, regardless of how many times the class has been instantiated. Local variables store temporary state inside a method. Parameters are variables that provide extra information to a method; both local variables and parameters are always classified as "variables" (not "fields"). When naming your fields or variables, there are rules and conventions that you must follow.

The eight primitive data types are: int, double, boolean, byte, short, long, float, and char. The java.lang.String class represents character strings. A literal is the source code representation of a fixed value. An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.

Sorce: ORACLE

Java Tutorial 6: Variables-Primitive Data Types

2012-04-15T17:08:00.003+01:00

Primitive Data Types

The Java programming language is statically-typed, which means that all variables must first be declared before they can be used. This involves stating the variable's type and name, as you've already seen:

int gear = 1;

Doing so tells your program that a field named "gear" exists, holds numerical data, and has an initial value of "1". A variable's data type determines the values it may contain, plus the operations that may be performed on it. In addition to int, the Java programming language supports seven other primitive data types. A primitive type is predefined by the language and is named by a reserved keyword. Primitive values do not share state with other primitive values.

The eight primitive data types supported by the Java programming language are listed below, the first 3 is the most common ones and they are probably the ones that you’ll ever need.

int: The int data type is a 32-bit signed two's complement integer. It has a minimum value of -2,147,483,648 and a maximum value of 2,147,483,647 (inclusive). For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, use long instead.

double: The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.

boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.

byte: The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of int where their limits help to clarify your code; the fact that a variable's range is limited can serve as a form of documentation.

short: The short data type is a 16-bit signed two's complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters.

long: The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive). Use this data type when you need a range of values wider than those provided by int.

float: The float data type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead. Numbers and Strings covers BigDecimal and other useful classes provided by the Java platform.

char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).

In addition to the eight primitive data types listed above, the Java programming language also provides special support for character strings via the java.lang.String class. Enclosing your character string within double quotes will automatically create a new String object; for example, String s = "this is a string";. String objects are immutable, which means that once created, their values cannot be changed. The String class is not technically a primitive data type, but considering the special support given to it by the language, you'll probably tend to think of it as such. You'll learn more about the String class in Simple Data Objects

Default Values

It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style, so I am not going to go into them.

Literals

You may have noticed that the “new” keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation. As shown below, it's possible to assign a literal to a variable of a primitive type:

boolean result = true;

int i = 100000;

Character and String Literals

Literals of types char and String may contain any Unicode (UTF-16) characters. If your editor and file system allow it, you can use such characters directly in your code. If not, you can use a "Unicode escape" such as '\u0108' (capital C with circumflex), or "S\u00ED Se\u00F1or" (Sí Señor in Spanish). Always use 'single quotes' for char literals and "double quotes" for String literals. Unicode escape sequences may be used elsewhere in a program (such as in field names, for example), not just in char or String literals.

The Java programming language also supports a few special escape sequences for char and String literals: \b (backspace), \t (tab), \n (line feed), \f (form feed), \r (carriage return), \" (double quote), \' (single quote), and \\ (backslash).

Using Underscore Characters in Numeric Literals

In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example. to separate groups of digits in numeric literals, which can improve the readability of your code.

For instance, if your code contains numbers with many digits, you can use an underscore character to separate digits in groups of three, similar to how you would use a punctuation mark like a comma, or a space, as a separator.

long creditCardNumber = 1234_5678_9012_3456L;

long socialSecurityNumber = 999_99_9999L;

You can place underscores only between digits; you cannot place underscores in the following places:

At the beginning or end of a number

Adjacent to a decimal point in a floating point literal

Prior to an F or L suffix

In positions where a string of digits is expected

mmp669-Android Development

Java Tutorial 26: Annotations

Annotations

Documentation

Annotations Used by the Compiler

Annotation Processing

Java Tutorial 25: Nested Classes

Nested Classes

Why Use Nested Classes?

Static Nested Classes

Inner Classes

Inner Class Example

Local and Anonymous Inner Classes

Modifiers

Summary of Nested Classes

Java Encapsulation

Java Tutorial 24: More on Classes-Initializing Fields

Static Initialization Blocks

Initializing Instance Members

Summary of Creating and Using Classes and Objects

Java Tutorial 23: More on Classes-Understanding Instance and Class Members

Class Variables

Class Methods

Constants

The Bicycle Class

Java Tutorial 22: More on Classes-Controlling Access to Members of a Class

Java Tutorial 21: More on Classes-Using the this Keyword

Using this with a Field

Using this with a Constructor

Java Tutorial 20: More on Classes-Returning a Value from a Method

Returning a Value from a Method

Returning a Class or Interface

Java Tutorial 19: Object-Creating Objects

Creating Objects

Declaring a Variable to Refer to an Object

Instantiating a Class

Initializing an Object

Understanding constructors-How constructors differ from methods

Purpose and function

Signature differences

The use of "this"

The use of "super"

Compiler-supplied code

Inheritance

Summarizing the differences

Java Tutorial 18: Classes: Passing Information to a Method or a Constructor

Parameter Types

Arbitrary Number of Arguments

Parameter Names

Passing Primitive Data Type Arguments

Java Tutorial 17: Classes-Defining Methods and Providing Constructors for Your Classes

Defining Methods

Naming a Method

Providing Constructors for Your Classes

Java Tutorial 16: Classes-Declaring Classes and Declaring Member Variables

Declaring Classes

Declaring Member Variables

Types

Variable Names

Java Tutorial 15: Control Flow Statements- Branching Statements

The break Statement

The continue Statement

Summary of Control Flow Statements

Java Tutorial 14: Control Flow Statements- The for Statement

Java Tutorial 13: Control Flow Statements- The while and do-while Statements

How to use these tutorials

Java Tutorial 12: Control Flow Statements- The switch Statement

Using Strings in switch Statements

Java Tutorial 11: Control Flow Statements-The if-then and if-then-else Statements

The if-then Statement

The if-then-else Statement

Java Tutorial 10: Expressions, Statements, and Blocks

Expressions

Statements

Blocks

Java Tutorial 9: Operators-Equality, Relational, and Conditional Operators

The Equality and Relational Operators

The Conditional Operators

Another conditional operator is ?:,

The Type Comparison Operator instanceof

The `Bicycle` Class

Using `this` with a Field

Using `this` with a Constructor