二叉树中的 Java 泛型，类型不兼容错误

Question

I'm trying to implement a Binary Tree in java with Generics, i searched and i find this question: Implementing Binary Tree in Java with Generic Comparable<T> data? , but i couldn't resolve my doubts. So i have two classes,

BST_Tree<T> 

and

Node<T extends Comparable<T>> 

I want that my implementation can:

1. Take every type of Object and put it inside the field key in Node

2. Compare every node with the key field

This is the code:

public class Node < T extends Comparable < T >> {

    private T key;
    private Node left;
    private Node right;
    private Node p;

    public void setKey(T key) {
        this.key = key;
    }

    public T getKey() {
        return key;
    }

    public Node getLeft() {
        return left;
    }

    public Node getRight() {
        return right;
    }

    public void setLeft(Node left) {
        this.left = left;
    }

    public void setRight(Node right) {
        this.right = right;
    }

    public void setP(Node p) {
        this.p = p;
    }

    public boolean getBolCompMin(T key) {
        return this.key.compareTo(key) < 0;
    }
}

My Node class is suppose to extend Comparable in order to compare the key.

This is my tree:

public class BST_Tree < T > {

    private ArrayList < Node > nodes;
    private Node root;

    public BST_Tree(Node root) {
        this.root = root;
    }

    public void insertNode(T key) {
        Node z = new Node();
        z.setKey(key);
        Node x = this.root;
        Node y = new Node();

        while (x != null) {

            y = x;
            if (z.getBolCompMin(x.getKey())) {
                x = x.getLeft();
            } else {
                x = x.getRight();
            }
        }

        z.setP(y);

        if (z.getBolCompMin(y.getKey())) {
            y.setLeft(z);
        } else {

            y.setRight(z);
        }
    }
    public void InOderWalk(Node x) {
        if (x != null) {
            InOderWalk(x.getLeft());
            System.out.println(x.getKey());
            InOderWalk(x.getRight());
        }
    }

    public Node getRoot() {
        return root;
    }
}

My tree tries to set the key in node z but it fails. This is the error:

incompatible types: T cannot be converted to java.lang.Comparable

Thank you in advance!

Answer 1

Your

public class BST_Tree<T>

should be

public class BST_Tree<T extends Comparable<T>>

And every Node variable inside your BST_Tree and Node classes should be Node<T> .

This would ensure that you can only instantiate your BST_Tree class with element types that implement Comparable .