Q: Anonymous inner class extending outer class

[Innar Made]

Hello ranchers,

I just finished K&B chapter on inner classes. I was happily experimenting around by writing small test programs till I got stuck with this one.

public class OuterClass {

	class InnerClass extends OuterClass {
	}

	public static void main(String[] args) {
		OuterClass outerClass = new OuterClass();
		outerClass.new InnerClass(); // OK
		outerClass.new InnerClass() {}; // compile error, why?
	}

}

I don't understand why making outerClass.new InnerClass() {}; causes compile time error. The error message says something like "No enclosing instance of type OuterClass is available due to some intermediate constructor invocation". Can anyone please explain me this? Thanks

[Ankit Garg]

I think you know that you need an instance of OuterClass to instantiate InnerClass (as it is a non-static inner class). In the same way, to extend InnerClass, you need an instance of OuterClass to call the Constructor of InnerClass. Basically if I want to extend InnerClass, I would have to write a code like this

class InnerSub extends OuterClass.InnerClass {
	InnerSub() {
		new OuterClass().super();
	}
}

But since anonymous inner classes can't have constructors, so you can't create an anonymous inner class of a non-static inner class...

[Innar Made]

Hey Ankit,

Thanks for your reply. Just to confirm that I understood you correctly:
1. anonymous inner classes cannot define new constructors;
2. if inner class InnerClass extends outer class OuterClass then subclassing InnerClass by making an anonymous inner class fails because it cannot call the parent constructor of InnerClass as InnerClass doesn't have instance of OuterClass.

[Ankit Garg]

1. Anonymous inner classes can't have any constructor
2. It doesn't matter whether InnerClass extends OuterClass or not, you can't create an anonymous inner class of a non-static inner class...

[Innar Made]

Hey again, Ankit : )

Can you elaborate more about your statement?

2. It doesn't matter whether InnerClass extends OuterClass or not, you can't create an anonymous inner class of a non-static inner class...

Here I am making an anonymous inner class from a non-static inner class, it compiles and prints out "Greetings from inner class"

public class OuterClass {

	class InnerClass {
		public void doInnerClassStuff() {
			System.out.println("Greetings from inner class");
		}
	}

	public static void main(String[] args) {
		InnerClass innerClass = new OuterClass().new InnerClass() {}; // new anonymous inner class from a non-static inner class
		innerClass.doInnerClassStuff();
	}
}

Thanks for your patience.

[Rajeev Rnair]

Interesting discussion!

It seems that in your first code the Inner Class extends Outer Class. And you CANNOT create an "anonymous inner class" out of the "Inner Class".

In second case, he Inner Class does NOT extend the Outer Class. And you CAN create an "anonymous inner class" out of the "Inner Class".

[Ankit Garg]

Which version of JDK are you using?? Are you using Eclipse?? Your original code compiles fine on my machine (JDK 1.6 build 11). And what I wrote about creating anonymous inner class of non-static inner class was wrong (I need to brush up my inner classes again )...

[Innar Made]

Hey guys,

I am using JDK 1.6.0.18 with Eclipse Galileo. You are right, Ankit, compiling from command line it all works! I guess the problem is solved then