Getting hold of the outer class object from the inner class object

Question

I have the following code. I want to get hold of the outer class object using which I created the inner class object inner. How can I do it?

public class OuterClass {

    public class InnerClass {
        private String name = "Peakit";
    }

    public static void main(String[] args) {
        OuterClass outer = new OuterClass();
        InnerClass inner = outer.new InnerClass();
       // How to get the same outer object which created the inner object back?
        OuterClass anotherOuter = ?? ;

        if(anotherOuter == outer) {
             System.out.println("Was able to reach out to the outer object via inner !!");
        } else {
             System.out.println("No luck :-( ");
        }
    }
}

EDIT: Well, some of you guys suggested of modifying the inner class by adding a method:

public OuterClass outer() {
   return OuterClass.this;
}

But what if I don't have control to modify the inner class, then (just to confirm) do we have some other way of getting the corresponding outer class object from the inner class object?

Answer 1

Within the inner class itself, you can use OuterClass.this. This expression, which allows to refer to any lexically enclosing instance, is described in the JLS as Qualified this.

I don't think there's a way to get the instance from outside the code of the inner class though. Of course, you can always introduce your own property:

public OuterClass getOuter() {
    return OuterClass.this;
}

EDIT: By experimentation, it looks like the field holding the reference to the outer class has package level access - at least with the JDK I'm using.

EDIT: The name used (this$0) is actually valid in Java, although the JLS discourages its use:

The $ character should be used only in mechanically generated source code or, rarely, to access pre-existing names on legacy systems.

Answer 2

OuterClass.this references the outer class.

Answer 3

You could (but you shouldn't) use reflection for the job:

import java.lang.reflect.Field;

public class Outer {
    public class Inner {
    }

    public static void main(String[] args) throws Exception {

        // Create the inner instance
        Inner inner = new Outer().new Inner();

        // Get the implicit reference from the inner to the outer instance
        // ... make it accessible, as it has default visibility
        Field field = Inner.class.getDeclaredField("this$0");
        field.setAccessible(true);

        // Dereference and cast it
        Outer outer = (Outer) field.get(inner);
        System.out.println(outer);
    }
}

Of course, the name of the implicit reference is utterly unreliable, so as I said, you shouldn't :-)

Answer 4

The more general answer to this question involves shadowed variables and how they are accessed.

In the following example (from Oracle), the variable x in main() is shadowing Test.x:

class Test {
    static int x = 1;
    public static void main(String[] args) {
        InnerClass innerClassInstance = new InnerClass()
        {
            public void printX()
            {
                System.out.print("x=" + x);
                System.out.println(", Test.this.x=" + Test.this.x);
            }
        }
        innerClassInstance.printX();
    }

    public abstract static class InnerClass
    {
        int x = 0;

        public InnerClass() { }

        public abstract void printX();
    }
}

Running this program will print:

x=0, Test.this.x=1

More at: http://docs.oracle.com/javase/specs/jls/se7/html/jls-6.html#jls-6.6

Answer 5

Here's the example:

// Test
public void foo() {
    C c = new C();
    A s;
    s = ((A.B)c).get();
    System.out.println(s.getR());
}

// classes
class C {}

class A {
   public class B extends C{
     A get() {return A.this;}
   }
   public String getR() {
     return "This is string";
   }
}

Answer 6

if you don't have control to modify the inner class, the refection may help you (but not recommend). this$0 is reference in Inner class which tells which instance of Outer class was used to create current instance of Inner class.

Answer 7

/**
 * Not applicable to Static Inner Class (nested class)
 */
public static Object getDeclaringTopLevelClassObject(Object object) {
    if (object == null) {
        return null;
    }
    Class cls = object.getClass();
    if (cls == null) {
        return object;
    }
    Class outerCls = cls.getEnclosingClass();
    if (outerCls == null) {
        // this is top-level class
        return object;
    }
    // get outer class object
    Object outerObj = null;
    try {
        Field[] fields = cls.getDeclaredFields();
        for (Field field : fields) {
            if (field != null && field.getType() == outerCls
                    && field.getName() != null && field.getName().startsWith("this$")) {
                field.setAccessible(true);
                outerObj = field.get(object);
                break;
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return getDeclaringTopLevelClassObject(outerObj);
}

Of course, the name of the implicit reference is unreliable, so you shouldn't use reflection for the job.