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 ?

With *"How this code is implemented ?"*, you mean how Java internally implements inner classes? — m0skit0, Mar 18 '13 at 16:54
see this http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class — ITroubs, Mar 18 '13 at 16:57
@ITroubs that actually has nothing to do with this question. — m0skit0, Mar 18 '13 at 17:03
@m0skit0 yes this is my question. If my question is difficult to understand, feel free to improve it. — gontard, Mar 18 '13 at 17:03
@m0skit0 actually it has. I just forgot to mention that he has to look at the post by jrudolph. He explains how Java handles nonstatic inner classes by showing the generated bytecode — ITroubs, Mar 18 '13 at 17:05

score 11 · Accepted Answer · answered Mar 18 '13 at 16:56

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.

@m0skit0 yes. May be he was the sole to understand my english. — gontard, Mar 18 '13 at 17:01

score 1 · Answer 2 · answered Mar 18 '13 at 16:53

1

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

answered Mar 18 '13 at 16:53

Lee Meador

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score 1 · Answer 3 · answered Mar 18 '13 at 16:54

1

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.

answered Mar 18 '13 at 16:54

Chris Cooper

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I think the question is about **how** is this done internally. – m0skit0 Mar 18 '13 at 16:57

score 0 · Answer 4 · edited May 23 '17 at 12:11

0

public and private is a pointer issue, it's a compiler issue.

The question is one of the compiler enforcing the scope of a class/variable/method. Because the private method getEnclosing() falls with the the scope of InnerClass, it can be accessed throughout that class.

Note that pointers have nothing to do with the issue.
Using reflection you can still access private members of a class.

edited May 23 '17 at 12:11

Community

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answered Mar 18 '13 at 16:55

Johan

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I think he means the opposite: how `InnerClass` can access `EnclosingClass`. – m0skit0 Mar 18 '13 at 16:56
@m0skit0, yes, because innerinner is a part of inner, it can access anything around it, that makes sense. – Johan Mar 18 '13 at 17:04

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

4 Answers4