Java Tutorial 26: Annotations

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() { }

or

@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

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

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

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

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

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

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

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

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.