How java implement the access to the enclosing class from an inner inner class?

Question

I have created an inner class in an inner class :

public class EnclosingClass {

    public class InnerClass {
        private EnclosingClass getEnclosing() {
            return EnclosingClass.this;
        }

        public class InnerInnerClass {
            private InnerClass getEnclosing() {
                return InnerClass.this;
            }

            private EnclosingClass getEnclosingOfEnclosing() {
                return EnclosingClass.this;
            }
        }        
    }
}

I have been surprised that java allows the InnerInnerClass to access directly the EnclosingClass. How is this code implemented internally by Java?

The InnerInnerClass keeps two pointers (one on the InnerClass and the other on the EnclosingClass) or the InnerInnerClass access the EnclosingClass through the InnerClass ?

Answer 1

You just need to disassemble the resulting class with javap to see what's going on:

private EnclosingClass getEnclosingOfEnclosing();
  Code:
     0: aload_0
     1: getfield      #1                  // Field this$1:LEnclosingClass$InnerClass;
     4: getfield      #3                  // Field EnclosingClass$InnerClass.this$0:LEnclosingClass;
     7: areturn

So first it gets the instance of the directly enclosing class, then it gets the "top-level" enclosing class from that.

Answer 2

If the inner classes are not 'static', they contain references internally to the class in which they are contained.

Answer 3

Unless you make an inner class static, then yes, it does have a reference to the instance it exists within, and can reference it members (including private), the same goes for inner inner classes, inner inner inner classes and so on.