Two questions on inner classes in Java (class A { class B { } })

Question

Sorry for the bad title, but I couldn't think of a better one.

I'm having a class A and a class B which is kind of a sub class of A, like so:

(Is there actually a correct name for it? Isn't "sub class" reserved for inheritance?)

class A {
    int i = 0;
    class B {
        int j = 1;
    }
}

class Test {
    public static void main() {
        A a = new A();
        B b = a.new B();
        A c = ??? b ??? // get "a" back
    }
}

From B every property of A can be accessed, therefore both, a.i and b.i, return 0. Now, I'm wondering whether it's somehow possible to retrieve the original object of type A out of b, as b contains everything that a contains? Simple casting apparently doesn't do the trick.

Second one:

class A {

    void print() {
        System.out.println("This is class A.");
    }

    class B {
        void print() {
            // <--- How to access print() of class A (like this.A.print() or smth)? 
            System.out.println("This is class B.");
        }
    }
}

You could alternatively also provide me with some good resources on this topic, as I've been too stupid to find a good one so far.

Thanks in advance. :)

Answer 1

[Edit: My answer is appropriate for C# programmers, but I can't guarantee that its applicable to Java.]

B is an inner class, not a subclass of A. Additionally, B does not hold an instance of A, so your code as is cannot return any instance of A.

You need to restructure your classes as follows:

class A
{
    public class B
    {
       public A Parent;
       public B(A parent)
       {
          this.Parent = parent;
       }
    }
}

Now your B class has a field 'Parent' which returns its parent. You can use these classes as follows (this is C# syntax, because I don't know if Java has a different syntax for instantiating inner classes):

public static void Main(String[] args)
{
    A parent = new A();
    A.B child = new A.B(child);
    A backToParent = child.Parent;
}

Of course, creating your B class in this way seems little funny: technically, you can pass in any parent. It would probably be better to rewrite your A class with a method which returns a B:

class A
{        
    public class B
    {
       public A Parent;
       public B(A parent)
       {
          this.Parent = parent;
       }
    }

    public B getChild()
    {
        return new B(this);
    }
}

public static void Main(String[] args)
{
    A parent = new A();
    A.B child = A.getChild();
    A backToParent = child.Parent;
}

Answer 2

There doesn't seem to be a way to access the outer class from outside. But you can do it like this:

class A {
    int i = 0;
    class B {
        final A outer = A.this;
        int j = 1;
    }
}

class Test {
    public static void main() {
        A a = new A();
        A.B b = a.new B();
        A c = b.outer // get "a" back
    }
}

ClassName.this will be the instance of the outerclass associated with the instance of an inner class.

Answer 3

You can access it with the ParentClass.this syntax from within the inner class.

e.g.

public class Outter
{
    class Inner {
        public Outter getOutter()
        {
            return Outter.this;
        }
    }

    public Inner getInner(){
        return new Inner();
    }
}

class Runner{
    public static void main(String[] args){
        Outter out = new Outter(); 
        Outter.Inner inner = out.getInner();

        System.out.println(inner.getOutter().toString());
    }
}

Answer 4

this seemed to work for me

class A {
    int i = 0;
    class B {
        int j = 1;
    }
}

class Test {
    public static void main() {
        A a = new A();
        A.B b = a.new B();
        A c = (A)b.getClass().getDeclaredField("this$0").get(b);
    }
}