Inner class object

user_6747523 0
Viewed 107

I couldn't understand the behavior of the below code.

My expectation was that it should print 100 in both println() methods. Why the result is different?

If I uncomment getX() method in inner class, then the output of println() methods are the same which is 100.

I couldn't figure out the reasoning of different behavior. Please help me to understand.

public class Foo {

    public static void main(String[] args) {
        MyOuter outerObj = new MyOuter();
        MyOuter.MyInner innerObj = outerObj.new MyInner();

        innerObj.setX();

        System.out.println("x: " + innerObj.getX()); //x: 3
        System.out.println("x: " + outerObj.getX()); //x: 100
    }
}
class MyOuter {
    private int x = 3;

    public int getX() { return x; }

    class MyInner extends MyOuter {
        public void setX(){ x = 100; }
//      public int getX() { return x; }
    }
}
inner-classes java
1 Answers

From MyInner's perspective, there are two private int x fields in play here:

  • One field super.x that is inherited from MyOuter, but inaccessible because it is declared as private, and MyInner as an inheriting object cannot access this field.
  • One field MyOuter.this.x from the sorrounding MyOuter-instance (since MyInner is a (non-static) inner class, it is always bound to an instance of the surrounding MyOuter), that is accessible.

The method setX() in MyInner cannot access the inherited field super.x, thus it accesses the field MyOuter.this.x and sets its values to 100. A consecutive call on the surrounding MyOuter's getX() will return 100.

The call innerObj.getX() (which is inherited from MyOuter and can access super.x) returns the value of the inherited field super.x (still having its initial value of 3).

If we remove the extends MyOuter from and include the getX()-method in MyInner, the code behaves as expected.

The fact that MyInner cannot access the inherited field private x is confusing at first, but the behaviour is actually in line with the behaviour of the keyword protected in the context of static mehthods in inheriting classes as discussed in this post by Hariharan

user_42166410
Related