use of casting

[manish ahuja]

Hi All
I would like to know what is the use of casting
Say if we have Class A with 1 method public void meth1()
now i create a child class
Class B extends A
& add 1 more method public void meth2()

now in a main method i create
A x = new B();
now i can call only meth1() with the super class reference
I would like to know whats the use of the above approach
Can some one show me practical use of this approach. having a sub classobject in a super class reference

Rgrds
Manish

[Mark Vedder]

Originally posted by manish ahuja:

now i can call only meth1() with the super class reference

No; you can call meth1() from a B reference. Since B extends A, it inherits all of A's methods. B can either use them as they are written in A, or override them so they do something different. Create the following and then run the Pgm class:

public class A
{
    public void meth1()
    {
        System.out.println("Method 1 in A");
    }
}
public class B extends A
{
    public void meth2()
    {
        System.out.println("Method 2 in B");
    }
}
public class Pgm
{
    public static void main(String[] args)
    {
        A a = new A();
        B b = new B();
        b.meth1();
        b.meth2();
    }
}

I would like to know whats the use of the above approach
Can some one show me practical use of this approach. having a sub classobject in a super class reference

The Collection classes are a good example of a practical reason for using this approach. Remembering that all classes are implicated subclasses of the Object class, the Collection classes store objects of type Objects. You can then cast the Ojbect as it comes out of the Collection object so that you have the "ability" to call that objects true methods:

B b = new B();
ArrayList arrayList = new ArrayList();
arrayList.add(b);
...
//Other code (or different method)
...
B b2 = (B) arrayList.get(0);    //We (The programmer) know that the object in
                                //the ArrayList is really of type 'B', but the 
                                //ArrayList.get(index) method returns an object
                                //of type Object, so we cast it.

Without this ability, we would need thousands of different ArrayList classes to hold every conceivable type of Object. And then what would we do for the special objects we create that the developers of the JDK did not know about?
Take a look at the Campfire story How my Dog learned Polymorphism for some more info. And if that wets your appetite, pick up the book Head First Java � it gives one of the best explanations of polymorphism and casting I have ever read.

[Mark Vedder]

Oh, and if you want to see a test of Overriding a method, overide meth1() in B and run Pgm. Notice the diifference?

public class A
{
    public void meth1()
    {
        System.out.println("Method 1 in A");
    }
}
public class B extends A
{
    public void meth1()
    {
        System.out.println("Method 1 in B");
    }
    
    public void meth2()
    {
        System.out.println("Method 2 in B");
    }
}
public class Pgm
{
    public static void main(String[] args)
    {
        A a = new A();
        B b = new B();
        a.meth1();
        b.meth1();
        b.meth2();
    }
}

[Marilyn de Queiroz]

When you "get" something from an ArrayList, you get an Object back.

Object object = myList.get( args[0] );

Then you have to cast it to whatever it was that you put into the ArrayList.

String name = (String)myList.get( args[0] );