Please explain the output.

01. import Outer.Inner.InnerMost;
02. import Outer.*;
03.
04. class Outer {
05. int i;
06. class Inner {
07. Inner() {i++;}
08.
09. class InnerMost extends Inner {
10. InnerMost() {i++; System.out.println(i);}
11. }
12. }
13. interface InnerInterface {
14. void doSomething();
15. }
16. }
17.
18. public class Test implements InnerInterface{
19.
20. public void doSomething() {
21. System.out.println("done");
22. }
23.
24. public static void main(String[] args) {
25. Inner im = new Outer().new Inner().new InnerMost();
26. }
27. }


The output is 3, Why is it not 2??

First, understand that the instances of Inner and InnerMost share the instance field i with the enclosing instance of Outer. This is true no matter how many Inner or InnerMost instances we create.

Second, realize that whenever a class instantiated, the constructor of its superclass is called.

So first, the instance of Outer is created along with the i instance with a value of zero.
Then, the Inner instance is created. The Inner constructor increments i to 1.
Finally, the InnerMost instance is created, and before any of the commands in its constructor are executed, it calls the superclass constructor.
So i is now incremented twice more: once in the superclass constructor, and another time in the InnerMost constructor.