In Java, how do I access the outer class when I'm not in the inner class?

Solution 1:

The bytecode of the Outer$Inner class will contain a package-scoped field named this$0 of type Outer. That's how non-static inner classes are implemented in Java, because at bytecode level there is no concept of an inner class.

You should be able to read that field using reflection, if you really want to. I have never had any need to do it, so it would be best for you to change the design so that it's not needed.

Here is how your example code would look like when using reflection. Man, that's ugly. ;)

public class Outer {
    public static void foo(Inner inner) {
        try {
            Field this$0 = inner.getClass().getDeclaredField("this$0");
            Outer outer = (Outer) this$0.get(inner);
            System.out.println("The outer class is: " + outer);

        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

    public class Inner {
    }

    public void callFoo() {
        // The constructor of Inner must be called in 
        // non-static context, inside Outer.
        foo(new Inner()); 
    }

    public static void main(String[] args) {
        new Outer().callFoo();
    }
}

Solution 2:

There is no way, by design. If you need to access the outer class through an instance of the inner one, then your design is backwards: the point of inner classes is generally to be used only within the outer class, or through an interface.

Solution 3:

What's wrong with adding a getter when you need to access the outer class? That way you can control whether access is allowed or not.

Solution 4:

This, in fact, is a very good question if, for example, you need to be able to check if two distinct instances of an Innner class share the same instance of Outer class (== or equals depending on the context).

I'd suggest to make a general purpose interface (not absolutely required for named inner classes but can be "instancedof"/casted to):

public interface InnerClass<Outer> {
    Outer getOuter();
}

that can be applied to any named inner class.

Then you do something like:

class MyInnerClass implements InnerClass<Outer> {
    Outer getOuter() {
        return Outer.this;
    }
    // remaining implementation details
}

and this way you can access outer class from any inner class implementing InnerClass<Outer> interface (and check it actually implements it).

If your inner class is anonymous, you can only do (thanks to Rich MacDonald for its sample):

new InterfaceOrAbstractClass<Outer>() {
    Outer getOuter() { // super inefficient but this is the only way !
        return (Outer)getClass().getDeclaredField("this$0");
    }
    /* other methods */
}

but InterfaceOrAbstractClass must implements InnerClass<Outer> to be able to access getOuter() outside of the anonymous class body!

It would be so much easier if javac automatically implemented some kind of InnerClass<Outer> interface on ALL inner classes, and it could do that super efficiently even on anonymous classes (no sluggish introspection processing)!