Hidden fields though inheritance

问题

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

回答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).

回答2:

Operations are always on Objects. In both cases, p.aMethod() and c.aMethod(), object is child that is why it printed 6 in both cases. When you directly access variable directly it reads variable associated with left side.

回答3:

Because declaring a variable doesn't inherit. You have two copies of x in the class, one in parent namespace, one in child namespace.

来源：https://stackoverflow.com/questions/8647248/hidden-fields-though-inheritance

标签

java

oop

inheritance

scjp