abstract classes

[sreedhara satuluri]

it is seen and read in theory that abstract class cannot be instantiated but there is a program in which an abstract class is instantiated.

 
abstract class abstclass1
{
public abstract void add(int a, int b);
}
abstract class abstclass2 extends abstclass1
{
   public abstract void sub(int a, int b); 
}
class abstclass3 extends abstclass1
{
int a, b;
 
abstclass3()
{
this.a=0;
this.b=0;
}
//overridden method
public void add(int a, int b)
{
System.out.println(a+b);
}
// new method
public void sub(int a, int b)
{
System.out.println(a-b);
}
}
class abstclass4 extends abstclass2
{
int a, b;
abstclass4()
{
this.a=0;
this.b=0;
}
// overridden method
      public void add(int a, int b)
      { 
      System.out.println(a+b);
      }
//overridden method
public void sub(int a, int b)
{
System.out.println(a-b);
}
}
public class AbstractClass_Ex3 
{
   public static void main(String[] args)
   {
 abstclass1 obj = new abstclass1()
{
      public void add(int a,int b)
   {
System.out.println((a+b));
   }
   
};
// calling abstractclass1 methods        
obj.add(20,30);
//obj.sub(20,30); // can't call sub() method, bcoz not declared in //abstract  
                                                       //class abstractclass1
abstclass2 obj1 = new abstclass2()// instantiated here
{
    public void add(int a, int b)
  {
     System.out.println((a+b));
  }
  public void sub(int a, int b)
  {
System.out.println((a-b));
  }
};
// calling abstractclass2 methods        
obj1.add(20,30);
obj1.sub(20,30); 
 
abstclass3 st3 = new abstclass3();
st3.add(10,35);  // calling overridden method
        
abstclass4 st4 = new abstclass4();
st4.add(10,35);  // calling overridden method
st4.sub(50,30);  // calling overridden method
  }
}

please comment on this

[ EJFH: Added "CODE" tags. ]
[ March 17, 2005: Message edited by: Ernest Friedman-Hill ]

[Ernest Friedman-Hill]

You're not instantiating abstclass1 and abstclass2; you're instantiating anonymous inner classes that extend abstclass1 and abstclass2 and provide the missing methods. An anonymous class is a real class, and the compiler generates a .class file for it (in this case, likely called
AbstractClass_Ex3$1.class and AbstractClass_Ex3$2.class .) They just don't appear to have a name at the point of declaration. These anonymous classes are every bit as real as abstclass3 and abstclass4.

Note: when you post code, please use the UBB "code" tags. Below the text area where you type your message, there are a bunch of little buttons; one says "CODE". Use that one, and your code will appear formatted in your post.