Problem -- Virtual Method Invocation

[Niyas Ahmed Sheikh]

Hi,

Below is the sample coding I had written to test virtual method invocation:

While compiling, it didn't display any error and even when I run the program it displays the output, but the last line it's give error message:

  
class Parent
{
  public void method1()
  {
     System.out.println("Method1 of Parent class");
  }
 
  public void method2()
  {
    System.out.println("Method2 of Parent class");
  }
 
  public void method3()
  {
     System.out.println("Method3 of Parent class");
  }
 
}
 
class Child extends Parent
{
  public void method1()
  {
     System.out.println("Method1 of Child class");
  }
 
  public void method2()
  {
    System.out.println("Method2 of Child class");
  }
 
  public void method4()
  {
    System.out.println("Method4 of Child class");
  }
 
  public static void main(String args[])
  {
 
    System.out.println("----------------------");  
    System.out.println("Method1 Method2 Method3 Belongs to Parent Class");  
    System.out.println("----------------------");  
    System.out.println("Method1 Method2 in Child class Override the parent class & Method3 Belongs to Child 
 
Class");        System.out.println("----------------------");  
 
    System.out.println("----------------------");  
    System.out.println("Child c = new Child();");  
    System.out.println("----------------------");  
    
    Child c = new Child();
    c.method1();
    c.method2();
    c.method3();
    c.method4();  
 
    System.out.println("------------------------");  
    System.out.println("Parent p = new Parent();");  
    System.out.println("------------------------");  
 
    Parent p = new Parent();
    p.method1();
    p.method2();
    p.method3();
//    p.method4(); //won't work
    
    System.out.println("-------------------------");  
    System.out.println("Parent p1 = new Parent();");  
    System.out.println("-------------------------");  
 

    Parent p1 = new Child();
    p1.method1();
    p1.method2();
    p1.method3();
//    p1.method4(); //won't work
   
   if ( p1 instanceof Child)
   {
     Child c1 = (Child) p;
     c1.method4(); 
   }
   else
   System.out.println("Failed");
  
   
  }
}     

Output
-------

  
----------------------
Method1 Method2 Method3 Belongs to Parent Class
----------------------
Method1 Method2 in Child class Override the parent class & Method3 Belongs to Child Class
 
----------------------
Child c = new Child();
----------------------
Method1 of Child class
Method2 of Child class
Method3 of Parent class
Method4 of Child class
------------------------
Parent p = new Parent();
------------------------
Method1 of Parent class
Method2 of Parent class
Method3 of Parent class
-------------------------
Parent p1 = new Parent();
-------------------------
Method1 of Child class
Method2 of Child class
Method3 of Parent class
[B]Exception in thread "main" java.lang.ClassCastException: Parent
        at Child.main(Child.java:78)[/B]

[Steve Simon Joseph Fernandez]

Hi,

In the last block given below

 
   if ( p1 instanceof Child)
   {
     Child c1 = (Child) p;
     c1.method4(); 
   }

you're checking whether p1 is an instance of Child, but inside the condition, you're trying to cast p to a Child. Assigning a superclass object to a subclass reference is illegal. Although the compiler allows it, the JVM will report a ClassCastException.

_steve.