How many different versions of 'x' are accessible in (*)?

Question

This is a training exercise for understanding the workings of inner classes in Java. As the question states, how many different versions of x are accessible in (*)?

class Outer {     int x;      class Inner extends Outer {         int x;          void f(int x) {             (*)         }     } } 

I'm inclined to think that there are 3, namely: this.x, super.x and x but some of my peers seem to think that there are 4.

Which of us is confused? And can you explain?

Answer 1

There are 4, namely: x, this.x, super.x and Outer.this.x.

Consider the following:

public class Outer {      int x;      public static void main(String[] args) {         Outer outer = new Outer();         Outer.Inner inner = outer.new Inner();         outer.x = 3;         inner.x = 2;         inner.f(1);     }      class Inner extends Outer {         int x;          void f(int x) {             System.out.println(super.x);             System.out.println(x);             System.out.println(this.x);             System.out.println(Outer.this.x);         }     }  } 

This code will print

0 1 2 3 

showing 4 different values.

What's happening is the following:

1. The parent of the inner instance has an unitialized x variable. For an int, the default value is 0: this is super.x.
2. The method f is called with the argument 1: this is x.
3. The instance inner was set its x to 2 with inner.x = 2: this is this.x.
4. The outer instance, which is Outer.this was set its x value to 3: this is Outer.this.x.

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The trick here is that Inner is both an inner class (so it has an enclosing Outer instance) and a subclass (so it has a parent Outer instance), and those two Outer instances are not the same.

Answer 2

There are four of them:

• Outer.this.x for the Outer class property
• this.x for the Inner class property
• super.x for the super type Outer class property
• x for the method argument