How to implement a tree data-structure in Java?

Question

Is there any standard Java library class to represent a tree in Java?

Specifically I need to represent the following:

• The sub-tree at any node can have

Answer 1

I wrote a small "TreeMap" class based on "HashMap" that supports adding paths:

import java.util.HashMap;
import java.util.LinkedList;

public class TreeMap<T> extends LinkedHashMap<T, TreeMap<T>> {

    public void put(T[] path) {
        LinkedList<T> list = new LinkedList<>();
        for (T key : path) {
            list.add(key);
        }
        return put(list);
    }

    public void put(LinkedList<T> path) {
        if (path.isEmpty()) {
            return;
        }
        T key = path.removeFirst();
        TreeMap<T> val = get(key);
        if (val == null) {
            val = new TreeMap<>();
            put(key, val);
        }
        val.put(path);
    }

}

It can be use to store a Tree of things of type "T" (generic), but does not (yet) support storing extra data in it's nodes. If you have a file like this:

root, child 1
root, child 1, child 1a
root, child 1, child 1b
root, child 2
root, child 3, child 3a

Then you can make it a tree by executing:

TreeMap<String> root = new TreeMap<>();
Scanner scanner = new Scanner(new File("input.txt"));
while (scanner.hasNextLine()) {
  root.put(scanner.nextLine().split(", "));
}

And you will get a nice tree. It should be easy to adapt to your needs.

Answer 2

In the past I have just used a nested map for this. This is what I use today, it is very simple but it fits my needs. Maybe this will help another one.

import com.fasterxml.jackson.annotation.JsonValue;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

/**
 * Created by kic on 16.07.15.
 */
public class NestedMap<K, V> {
    private final Map root = new HashMap<>();

    public NestedMap<K, V> put(K key) {
        Object nested = root.get(key);

        if (nested == null || !(nested instanceof NestedMap)) root.put(key, nested = new NestedMap<>());
        return (NestedMap<K, V>) nested;
    }

    public Map.Entry<K,V > put(K key, V value) {
        root.put(key, value);

        return (Map.Entry<K, V>) root.entrySet().stream().filter(e -> ((Map.Entry) e).getKey().equals(key)).findFirst().get();
    }

    public NestedMap<K, V> get(K key) {
        return (NestedMap<K, V>) root.get(key);
    }

    public V getValue(K key) {
        return (V) root.get(key);
    }

    @JsonValue
    public Map getRoot() {
        return root;
    }

    public static void main(String[] args) throws Exception {
        NestedMap<String, Integer> test = new NestedMap<>();
        test.put("a").put("b").put("c", 12);
        Map.Entry<String, Integer> foo = test.put("a").put("b").put("d", 12);
        test.put("b", 14);
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(test));

        foo.setValue(99);
        System.out.println(mapper.writeValueAsString(test));

        System.out.println(test.get("a").get("b").getValue("d"));
    }
}

Answer 3

There is actually a pretty good tree structure implemented in the JDK.

Have a look at javax.swing.tree, TreeModel, and TreeNode. They are designed to be used with the JTreePanel but they are, in fact, a pretty good tree implementation and there is nothing stopping you from using it with out a swing interface.

Note that as of Java 9 you may wish not to use these classes as they will not be present in the 'Compact profiles'.

Answer 4

What about this?

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;

/**
  * @author ycoppel@google.com (Yohann Coppel)
  * 
  * @param <T>
  *          Object's type in the tree.
*/
public class Tree<T> {

  private T head;

  private ArrayList<Tree<T>> leafs = new ArrayList<Tree<T>>();

  private Tree<T> parent = null;

  private HashMap<T, Tree<T>> locate = new HashMap<T, Tree<T>>();

  public Tree(T head) {
    this.head = head;
    locate.put(head, this);
  }

  public void addLeaf(T root, T leaf) {
    if (locate.containsKey(root)) {
      locate.get(root).addLeaf(leaf);
    } else {
      addLeaf(root).addLeaf(leaf);
    }
  }

  public Tree<T> addLeaf(T leaf) {
    Tree<T> t = new Tree<T>(leaf);
    leafs.add(t);
    t.parent = this;
    t.locate = this.locate;
    locate.put(leaf, t);
    return t;
  }

  public Tree<T> setAsParent(T parentRoot) {
    Tree<T> t = new Tree<T>(parentRoot);
    t.leafs.add(this);
    this.parent = t;
    t.locate = this.locate;
    t.locate.put(head, this);
    t.locate.put(parentRoot, t);
    return t;
  }

  public T getHead() {
    return head;
  }

  public Tree<T> getTree(T element) {
    return locate.get(element);
  }

  public Tree<T> getParent() {
    return parent;
  }

  public Collection<T> getSuccessors(T root) {
    Collection<T> successors = new ArrayList<T>();
    Tree<T> tree = getTree(root);
    if (null != tree) {
      for (Tree<T> leaf : tree.leafs) {
        successors.add(leaf.head);
      }
    }
    return successors;
  }

  public Collection<Tree<T>> getSubTrees() {
    return leafs;
  }

  public static <T> Collection<T> getSuccessors(T of, Collection<Tree<T>> in) {
    for (Tree<T> tree : in) {
      if (tree.locate.containsKey(of)) {
        return tree.getSuccessors(of);
      }
    }
    return new ArrayList<T>();
  }

  @Override
  public String toString() {
    return printTree(0);
  }

  private static final int indent = 2;

  private String printTree(int increment) {
    String s = "";
    String inc = "";
    for (int i = 0; i < increment; ++i) {
      inc = inc + " ";
    }
    s = inc + head;
    for (Tree<T> child : leafs) {
      s += "\n" + child.printTree(increment + indent);
    }
    return s;
  }
}

Answer 5

I wrote a little library that handles generic trees. It's much more lightweight than the swing stuff. I also have a maven project for it.

Answer 6

For example :

import java.util.ArrayList;
import java.util.List;



/**
 * 
 * @author X2
 *
 * @param <T>
 */
public class HisTree<T> 
{
    private Node<T> root;

    public HisTree(T rootData) 
    {
        root = new Node<T>();
        root.setData(rootData);
        root.setChildren(new ArrayList<Node<T>>());
    }

}

class Node<T> 
{

    private T data;
    private Node<T> parent;
    private List<Node<T>> children;

    public T getData() {
        return data;
    }
    public void setData(T data) {
        this.data = data;
    }
    public Node<T> getParent() {
        return parent;
    }
    public void setParent(Node<T> parent) {
        this.parent = parent;
    }
    public List<Node<T>> getChildren() {
        return children;
    }
    public void setChildren(List<Node<T>> children) {
        this.children = children;
    }
}