Display tree hierarchy with their values in Java

Question

I have to create a tree structure with a parent node having many children, and each child can also have their children. Each node will be identified by their unique id and name.

So when I enter any id from the hierarchy, all node-ids and their child node-ids with their names should get printed. How can I achieve this(any sample code would help) and which collection should be used from collection framework or do I have to use plain data structures in Java?

Answer 1

One possible way is to create your own tree structure (node with children) and wrap it into another structure that tracks nodes' ids. Something like that:

public class Tree<I, A> {
    private final HashMap<I, Node<I, A>> map = new HashMap<>();
    private final Node<I, A> root;

    public Tree(I id, A value) {
        root = new Node<>(id, value);
        map.put(id, root);
    }

    public void addChild(I parentId, I id, A value) {
        Node<I, A> parent = map.get(parentId);
        Node<I, A> child = new Node<>(id, value);
        parent.children.add(child);
        map.put(id, child);
    }

    public A getById(I id) {
        return map.get(id).value;
    }

    public String subtreeToString(I id) {
        return map.get(id).toString();
    }

    private static class Node<I, A> {
        private final I id;
        private final A value;
        private final ArrayList<Node<I, A>> children = new ArrayList<>();

        private Node(I id, A value) {
            this.id = id;
            this.value = value;
        }

        private void print(int depth, PrintWriter pw) {
            for (int i = 0; i < depth; i++) {
                pw.print("\t");
            }
            pw.println("[" + id + ", " + value + "]");
            for (Node<I, A> child : children) {
                child.print(depth + 1, pw);
            }
        }

        @Override
        public String toString() {
            StringWriter writer = new StringWriter();
            print(0, new PrintWriter(writer));
            return writer.toString();
        }
    }
}

Usage:

Tree<Integer, String> tree = new Tree<>(1, "Bob");

tree.addChild(1, 2, "John");
tree.addChild(1, 3, "James");
tree.addChild(2, 4, "David");
tree.addChild(2, 5, "Alice");

System.out.println(tree.subtreeToString(1));
System.out.println(tree.subtreeToString(2));

Output:

[1, Bob]
    [2, John]
        [4, David]
        [5, Alice]
    [3, James]

[2, John]
    [4, David]
    [5, Alice]

Answer 2

您将必须制作自己的树结构，并且在打印时，只需在父节点上调用toString（），然后从该节点上在子节点上调用toString（）等。

Answer 3

There might be some structure ready for the task, but on the other hand: this task is very straightforward to do yourself – just do a recursive in-order walk down the tree:

1. Start at the root node
2. If there is a left node: enter it and go to 2.
3. Else: print the content.
4. If there is a right node: enter it and go to 2.
5. Else: return

Good luck.