confusion on overriding variables

[Soni Prasad]

  
class A{
  int a = 10;
  void abc(){
     System.out.println(a);
  }
}
public class testOver extends A{
  int a = 20;
  
  public static void main(String[] args){
     testOver o = new testOver();
     A p = new A();
     p.abc(); 
     o.abc(); //1
 
     A q = new testOver();
     q.abc();  
 
     System.out.println(p.a+" "+o.a+ " "+q.a); 
  }
}
 

output is
10
10
10
10 20 10

I am confused that as variables are not overriden, at line number 1 reference type of object o is testOver then why output from line 1 is not 20? As abc() is being called on testOver reference then variable of testOver class should be printed which is 20. Please explain...

[Soni Prasad]

I am eagerly waiting for some explanation. Please explain...

[Rohan Kayan]

class A
{  int a = 10;
  void abc()
    {   
  System.out.println(a); 
 }
}
public class testOver extends A
{  
int a = 20;    
public static void main(String[] args)
{     testOver o = new testOver();
     A p = new A();     
     p.abc(); 
     o.abc(); //1    
     A q = new testOver();   
     q.abc();        
     System.out.println(p.a+" "+o.a+ " "+q.a);   
}
}  

As we are calling the method which is in class A only variable those are in the class A or in the parent class of A will only be accessible from that method . That is why it is printing 10 10 10 .

[Rajee Dhana]

try to override the method abc() in testOver class, and see the difference.

[Andrey Khashin]

Originally posted by Soni Prasad:

  
class A{
  int a = 10;
  void abc(){
     System.out.println(a);
  }
}
public class testOver extends A{
  int a = 20;
  
  public static void main(String[] args){
     testOver o = new testOver();
     A p = new A();
     p.abc(); 
     o.abc(); //1
 
     A q = new testOver();
     q.abc();  
 
     System.out.println(p.a+" "+o.a+ " "+q.a); 
  }
}
 

output is
10
10
10
10 20 10

I am confused that as variables are not overriden, at line number 1 reference type of object o is testOver then why output from line 1 is not 20? As abc() is being called on testOver reference then variable of testOver class should be printed which is 20. Please explain...

Look here for the same problem solution
http://www.coderanch.com/t/248529/java-programmer-SCJP/certification/accessing-variables-methods-polymorphic-way

[Jeff Hu]

Originally posted by Soni Prasad:

  
class A{
  int a = 10;
  void abc(){
     System.out.println(a);
  }
}
public class testOver extends A{
  int a = 20;
  
  public static void main(String[] args){
     testOver o = new testOver(); //1
     A p = new A(); //2
     p.abc(); //3
     o.abc(); //4
 
     A q = new testOver(); //5
     q.abc();  //6
 
     System.out.println(p.a+" "+o.a+ " "+q.a); //7 
  }
}
 

Let's go through our code, see its run-time behavour. testOver IS-A A. Check instance variable a, we noticed that it has default access. method abc() is in class A.
//1 testOver Object o is created with a = 20, BUT a = 10 in Class is also there on the stack.
//2 A Object p is created with a = 10.
//3 p invokes abc(). this is clear i guess.. printout 10
//4 o invokes abc(). Since there is no overriden abc(), it can only see instance var a belonging to the class which contains abc() method... printout 10
//5 reference variable q of type A is created refering to object of type testOver.
//6 q invokes abc(). For the same reason as //4, it prints out 10
//7 access instance variable a. We only look at object referece type, so p is reference type A -> p.a = 10; o is reference type testOver -> o.a = 20; q is reference type A -> q.a =10.


output is
10
10
10
10 20 10

If we modify testOver class (add method abc() to overide its super abc()), do similar analysis before.

public class testOver extends A
{  
int a = 20;    
 
void abc()
    {   
  System.out.println(a); 
 }
public static void main(String[] args)
{     testOver o = new testOver();
     A p = new A();     
     p.abc(); 
     o.abc(); //1    
     A q = new testOver();   
     q.abc();        
     System.out.println(p.a+" "+o.a+ " "+q.a);   
}

We have output
10
20
20
10 20 10

[Soni Prasad]

Thanx alot! now its clear to me

soni.