How to inherit an inner class?

Consider the following example,

class A {
 class B {
     B(int i) {
     ...
     }
 }
 ...
}

Sub-classing B inside class A is very simple because both class B and its sub-class, say C, require the instance of class A and can be done as follows.

class A {
 class B {
     B(int i) {
     ...
     }
 }
 class C extends B {
     C(int i) {
         super(i);
         ...
     }
 }
 ...
}

However, if you want to sub-class B outside class A, as follows,

class C extends A.B {
 C(int i) {
     super(i);
     ...
 }
 ...
}

you will get a compiler error as "an enclosing instance that contains A.B is required".

In order to solve this problem, you need to get an instance of A, as follows,

class C extends A.B {
 C(A a, int i) {
     a.super(i);
 }
}

Here is the compelete code.

public class A {
 class B {
     int i;
     B(int i) {
         this.i = i;
     }
 }
}
class C extends A.B {
 C(A a, int i) {
     a.super(i);
 }
 public static void main(String[] args) {
     A a = new A();
     C c = new C(a, 10);
     System.out.println("c.i = " + c.i);
 }
}