Java Inner class compilation error

Question

I'm trying to instantiate a public non-static inner class but I keep getting a compilation error.

I have 2 classes: a tree class and a main class.

I'm ultimately trying to instantiate an array of points, but for the sake of this question, I'm trying to just instantiate a Point object.

The compilation error says that

Point can't be resolved to a type

What am I doing wrong?

Tree.java

public class Tree<T> {  
    public class Point<T> {
        public T position[];
    }
}

Main.java

public class Main {
    public static void main(String args[]) {
        Point<Double> point = new Tree<Point<Double>>().new Point<Double>();
    }
}

Answer 1

Just leave out the T parameter when declaring the inner class.

Just use T inside, but don't try to redefine a type parameter with the same name T.

Answer 2

As suggested in the comments:

Tree<Double>.Point<Double> point = new Tree<Double>().new Point<Double>();

but it looks like a little bit too redundant don't you think?

you can remove the generic part of point

public class Tree<T> {
    public class Point {
        public T position[];
    }
}


Tree<Double>.Point point = new Tree<Double>().new Point();

Answer 3

I suspect you don't want to have two different type parameters for Tree and Point . The way you wrote it, the T of Point shades the T of Tree , so the parameters are actually different. If you want them to be the same, remove the T of the inner class. This way the T of outer class can be used in the inner class as well.

public class Tree<T> {  
    public class Point {
        public T position[];
    }

    public static void main(String args[]) {
        Tree<Double>.Point point = new Tree<Double>().new Point();
    }
}

If you really want to have two different T s, you can instantiate a Point like this. But then I highly recommend to rename one of the type parameters to prevent confusion.

Tree<Object>.Point<Double> point = new Tree<>().new Point<>();

Edit : According to the comments:

Tree<Tree<?>.Point<Double>>.Point<Double> point = new Tree<Tree<?>.Point<Double>>().new Point<>();

Edit : According to further comments

Since you want a Tree to contain always Points, there is no use in using generics here. It is a little bit dependent on what you want to do with this class, whether to remove the type parameter of Tree of the one of Point .

Possibility 1 removing type parameter of Tree

public class Tree {  
    public class Point<T> {
        public T position[];
    }
}

You can go with that if you want to return a Point from the methods in the Tree

Possibility 2 removing type parameter of Point

public class Tree<T> {  
    public class Point {
        public T position[];
    }
}

You should go with that if you want the methods in Tree to return objects of type T