polymorphism

[cchetan jain]

class A
{
int x=5;
}

class B extends A
{
int x=6;
}



class covtest
{

public A getObject()
{
System.out.println("its A from covtest");
return new A();
}


public static void main(String arg[])
{
covtest t = new subcovtest();
System.out.println(t.getObject().x); // *
}
}



class subcovtest extends covtest
{
public B getObject()
{
System.out.println("its B from subcovtest");
return new B();
}
}
the output is:
its B from subcovtest
5;
expected output is:
its B from subcovtest
6;
please explain,why?

[Frank Kellinghusen]

Your code us unreadable:

http://faq.javaranch.com/java/UseCodeTags

[cchetan jain]

  class A 
{ 
int x=5; 
} 

class B extends A 
{ 
int x=6; 
} 



class covtest 
{ 

public A getObject() 
{ 
System.out.println("its A from covtest"); 
return new A(); 
} 


public static void main(String arg[]) 
{ 
covtest t = new subcovtest(); 
System.out.println(t.getObject().x); // * 
} 
} 



class subcovtest extends covtest 
{ 
public B getObject() 
{ 
System.out.println("its B from subcovtest"); 
return new B(); 
} 
} 
the output is: 
its B from subcovtest 
5; 
expected output is: 
its B from subcovtest 
6; 
please explain,why?  
 
  

[Campbell Ritchie]

Frank Kellinghusen wrote:Your code us unreadable:

http://faq.javaranch.com/java/UseCodeTags

It's not much better without indentation.

[Campbell Ritchie]

Frank Kellinghusen, welcome to JavaRanch

Cchetan Jain, remember that polymorphism doesn't apply to fields, nor to static members.

[shivendra tripathi]

t.getObject() will be decided at run time while t.getObject().x will be decided at compile time. Because polymorphism is appicable only for instance mehtod.

[cchetan jain]

hello shivendra tripathi..please explain what is the concept behind the t.getObject().x;
when t.getObject() returns B then simply it must return 6..i m confused with this..

[Rob Spoor]

I repeat:

Campbell Ritchie wrote:remember that polymorphism doesn't apply to fields, nor to static members.

When accessing fields or static methods, the JVM uses the reference type. Since t's reference type is covtest, it's getObject() method return type is A, not B*. Therefore, A's field x is used.

* The actual type (as returned by getClass()) is B, but the declared type is A.

[Campbell Ritchie]

Rob Prime wrote:* The actual type (as returned by getClass()) is B, but the declared type is A.

I thought it would be something like that.

Rob is very good about that sort of thing. Thank you, Rob

[cchetan jain]

thanks for your wisdom...

[Campbell Ritchie]

You're welcome