Java类继承自引用类

Question

I'm trying to implement a simple Tree in a java class. However, I want another class to extend this Tree Class.

I'm making the tree recursive like this:

class Tree<T> {
    public T data;
    public List<Tree<T>> children;

    public Tree(T data){
        this.data = data;
    }
}

However, say I make a class like this:

class WeightedTree<T> extends Tree<T>

it's children are all regular trees, not Weighted Trees. Is there a way of accomplishing this with OOP principles?

Answer 1

You can do something like this:

class Tree<T, SELF extends Tree<T, SELF>> {
    public T data;
    public List<SELF> children;
}

class WeightedTree<T> extends Tree<T, WeightedTree<T>> {}

If you need to instantiate Tree , you can use a wildcard:

Tree<String, ?> tree = new Tree<>();

Or you can make an abstract base class with a self type and separate subclasses for Tree and WeightedTree .

Answer 2

/* Package */ abstract class TreeBase<T, TREE_T extends TreeBase<T, TREE_T>> {
    public T data;
    public List<TREE_T> children;

    public TreeBase(T data) {
        this.data = data;
    }
}

public class Tree<T> extends TreeBase<T, Tree<T>> {
    public Tree(T data) {
        super(data);
    }
}

public class WeightedTree<T> extends TreeBase<T, WeightedTree<T>> {
    public WeightedTree(T data) {
        super(data);
    }
}