Reference anonymous object from within a nested anonymous class

Question

Is it possible to do something like this (I use initializer blocks to shorten the example)

new A() {{
  new B() {{
    method(outer.this);
  }}
}}

Where I supply the this of the outer object as a parameter to the method call within the second anonymous class? I cannot use A.this, this gives a compile error.

Note: the given code does not compile, it should only illustrate what I'm trying to achieve.

Edit: example that lies closer to the actual use case:

public class Outer {

  public SomeBean createBean() {
    return new SomeBean() {

      private final Object reference = new SomeClass() {

        @Override
        public void notify() {
          Outer.callback(/*what goes here???*/);
        }
      };

      //Methods...
    };
  }

  public static void callback(final SomeBean bean) {
    // do stuff with bean
  }
}

And the compile error I get is just that I'm not providing the correct argument to callback, as I don't know how to reference the SomeBean subclass...

does it work when you put the code inside a method? Inside the initializers you are trying to reference an object which isn't created yet. — Jens Schauder
– Jens Schauder, Commented Jan 2, 2012 at 16:15
I don't recommend unsafe publication of this. Also, can you please show us the compiler error? — mre
– mre, Commented Jan 2, 2012 at 16:20
I elaborated the problem a bit more in the original question. — rvdp
– rvdp, Commented Jan 3, 2012 at 18:30

Tom Hawtin - tackline · Accepted Answer · 2012-01-02 16:46:10Z

2

If you really must, I guess this should work.

new A() {
    {
        new B() {{
            method();
        }};
    }
    private void method() {
        method(this);
    }
}

(Historical note: With -target 1.3 or earlier, this should NPE.)

If you don't need the exact type of the A inner class.

new A() {
    {
        new B() {{
            method(a);
        }};
    }
    private A a() {
        return this;
    }
}

edited Jan 2, 2012 at 16:46

answered Jan 2, 2012 at 16:40

Tom Hawtin - tackline

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2 Comments

rvdp Over a year ago

The first suggestion is along the lines of what I've currently done, it works, but I was wondering whether it is possible to directly call the needed method instead of providing a "proxy-method", I'll keep this solution if the direct call isn't possible though.

rvdp Over a year ago

I'll accept this one as it is the solution I also came up with myself and I don't think there really is a better way.

Aaron J Lang · Accepted Answer · 2012-01-03 23:36:08Z

1

@TomHawtin 's answer is good, mine is quite similar. I would do this:

new A() {
    private final A anon = this;
    /*init*/ {
        new B() {
            /*init*/ {
                method(anon);
            }
        }
    }
}

~~This will probably give you slightly better performance than calling a method to get your A instance.~~ The main benefit is IMO this is easier to read/maintain.

Edit: @Tomas 's answer is also very similar, but required that you keep a reference to your new A object in the outer-outer class, where it might not be needed.

In light of op's edit:

public SomeBean createBean() {
    SomeBean myBean = new SomeBean() {
        private final Object reference = new SomeClass() {
            @Override
            public void notify() {
                Outer.callback(/*what goes here???*/);
            }
        };
        //Methods...
    };
    return myBean;
}

FYI obj.notify() is a final method in Object, you can't override it. JavaDoc here

edited Jan 3, 2012 at 23:36

answered Jan 2, 2012 at 17:01

Aaron J Lang

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3 Comments

Tom Hawtin - tackline Over a year ago

I'm not sure what you mean by your comment. / The field solution is slightly more concise, although it does add an extra field, probably giving slightly worse performance. I think the private field will cause a synthetic accessor method to be generated.

Aaron J Lang Over a year ago

I can't claim any intimate knowledge of the compiler, I was under the impression that field-access was slightly faster than trivial method invokation. I'll edit my answer.

rvdp Over a year ago

notify was just an example name, I didn't think of the method in Object while typing it.

Tomas Narros · Accepted Answer · 2012-01-02 16:38:35Z

Consider refactoring your code, as the code you are proposing doesn't seem too logical, and for sure not easy to handle or mantain. You could do a pretty similar structure (subclassing two different types and using one from another) with something like this:

//declaring a variable as final enables accessing it from the B subtype if declared at the same scope (method)
final A myA=new A() {
   //personalized content for type A
};
//then, declare the B subtype, using the final myA var as you need, at the constructor 
B myB=new B() {{
   method(myA);
}};

This, in case you need to overwrtie both classes, as you exposed. Maybe a little rethink could save you from these anonymous classes (or at least, from some of them).

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