When I assign new value to my binary search tree the same way my input method function do, program report error say cannot be referenced from static context. (this line a.root.left = new Node("dd",22) say Node is not static

this is my program:

package trylang;

import java.util.Arrays;

public class tryBSTnode<Key extends Comparable<Key>, Value> {
    private Node root;
    private class Node{
        private Key key;
        private Value value;
        private Node left, right;
        public Node(Key key, Value val){
            this.key = key;
            this.value = val;
        }
    }
    public tryBSTnode(){

    }

    public void put(Key key, Value val){
        root = put(root,key,val);
    }

    public Node put(Node nd, Key key, Value val){
        if (nd == null) return new Node(key, val);
        int cmp = key.compareTo(nd.key);
        if (cmp>0) nd.right = put(nd.right,key,val);
        else if (cmp<0) nd.left = put(nd.left, key, val);
        else nd.value = val;
        return nd;
    }
    public static void main(String[] args) {
        tryBSTnode<String, Integer> a = new tryBSTnode<String, Integer>();
        a.put("aa",20);
        a.put("bb",67);
        a.put("cc",67);
        a.root.left = new Node("dd",22); //this line got problem
    }
}

The put method logic is that when node is null, we can assign a new node to it.

 public Node put(Node nd, Key key, Value val){
        if (nd == null) return new Node(key, val);
        int cmp = key.compareTo(nd.key);
        if (cmp>0) nd.right = put(nd.right,key,val);
        else if (cmp<0) nd.left = put(nd.left, key, val);
        else nd.value = val;
        return nd;
    }

The tree structure would be like root(aa,20)->right(bb,67)->right(cc,67), and run the put method do not meet any error. The left side of root is null. However, when I want to assign a new node to left side of tree, itellij say this line a.root.left = new Node("dd",22) cannot be referenced from static context. Why this line can work in put method, but have error in the main method?

CodePudding user response：

Node is an inner class, having a tryBSTNode.this besides this, with which it can access members - fields or methods - of a tryBSTNode object.

As that binding of Node is not intended, make it static:

private static class Node {

Or move the class outside tryBSTNode; you are allowed to have additional non-public classes in one source.

Or make a separate source.

You should have named your main class TryBSTNode. It also looks a bit like a unit test, Test Driven Development. You might look into that, eventually.

CodePudding user response：

As the error says; you can't access non-static members (Node) from statics context (the main method). Put the Node class out of tryBSTNode class and it works.