Question

I am new to Java coming from a PHP background so sorry if this maybe obvious. I’m trying to implement a binary tree class and I’ve created an ADT like so:

public abstract class BTree<T> {
    private T value;
    private BTree<T> leftChild;
    private BTree<T> rightChild;
    private BTree<T> parent;
    public BTree<T> getLeftChild() { return this.leftChild; }
    ....
} 

Then I have another class that extends this like so:

public class BIntTree extends BTree<Integer> {
}

However I want to be able to within BIntTree to have a method where I can call this.getLeftChild(); and get an instance of BIntTree back rather than a instance of BTree<Integer>

Is this possible with some way of defining the generic class / method or do I have to explicitly type cast it after I used this.getLeftChild() or even override the superclass method?

My current solution is to explicitly typecast it in the BIntTree method with BIntTree b=(BIntTree) this.getLeftChild(); which seems untidy to me.

Also I’m not so sure what would happen if I had that type casting defined and getLeftChild() returned null, would an exception be thrown? If so how do I cure this given that null is also a valid value if exist?

Answer 1

It is possible to have a self-typed class, but you have to wonder why you want it. Why do you need to know that it is a BIntTree rather than a BTree<Integer>? What have you gained with your use of generics?

Anyway, you could do something like this:

public abstract class BTree<V, T extends BTree<V, T>> {
    public T getLeft() {...}
    public T getRight() {...}
    public V getValue() {...}
}

Then your BIntTree would be

public class BIntTree extends BTree<Integer, BIntTree> {
    //...
}

Edit

With regards to your question about casting null, there is really nothing preventing you from testing the behaviour yourself. But to answer your question, it is safe to cast null to any type.