Question

In the following code example:

class Parent { 
    int x =5;
    public Integer aMethod(){

        System.out.print("Parent.aMthod ");
        return x;
    }
}

class Child extends Parent {
    int x =6;
    public Integer aMethod(){
        System.out.print("Child.aMthod "); 
        return x;
    }
}


class ZiggyTest2{

    public static void main(String[] args){

        Parent p = new Child();
        Child c = new Child();

        System.out.println(p.x + " " + c.x);

        System.out.println(p.aMethod() + "  \n");
        System.out.println(c.aMethod() + "  \n");
    }   
}

And the output:

5 6
Child.aMthod 6  

Child.aMthod 6

Why does p.aMethod() not print 6 when p.x prints 6?

Thanks

Edit

Oops a slight typo: The question should be “why does p.aMethod() not print 5 when p.x print 5” – Thanks @thinksteep

Answer 1

There’s no polymorphic resolution being done when you access class member fields (instance variables) like p.x. In other words, you’ll get the results from the class that’s known at compile time, not what is known at run time.

For method calls this is different. They are dispatched at run time to an object of the actual class the reference points to, even if the reference itself has a super type. (in the VM this happens via the invokevirtual opcode, see e.g. http://java.sun.com/docs/books/jvms/second_edition/html/Instructions2.doc6.html#invokevirtual).