Java中有一些树数据结构，比如JDK Swing中的DefaultMutableTreeNode, Stanford解析器包中的tree，以及其他一些玩具代码。但这些都不够，也不够小，不能用于一般用途。

Java tree项目试图在Java中提供另一种通用的树数据结构。这个和其他的区别是

Totally free. You can use it anywhere (except in your homework :P) Small but general enough. I put everything of the data structure in one class file, so it would be easy to copy/paste. Not just a toys. I am aware of dozens of Java tree codes that can only handle binary trees or limited operations. This TreeNode is much more than that. It provides different ways of visiting nodes, such as preorder, postorder, breadthfirst, leaves, path to root, etc. Moreover, iterators are provided too for the sufficiency. More utils will be added. I am willing to add more operations to make this project comprehensive, especially if you send a request through github.

    // TestTree.java
// A simple test to see how we can build a tree and populate it
//
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;

public class TestTree extends JFrame {

  JTree tree;
  DefaultTreeModel treeModel;

  public TestTree( ) {
    super("Tree Test Example");
    setSize(400, 300);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
  }

  public void init( ) {
    // Build up a bunch of TreeNodes. We use DefaultMutableTreeNode because the
    // DefaultTreeModel can use it to build a complete tree.
    DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
    DefaultMutableTreeNode subroot = new DefaultMutableTreeNode("SubRoot");
    DefaultMutableTreeNode leaf1 = new DefaultMutableTreeNode("Leaf 1");
    DefaultMutableTreeNode leaf2 = new DefaultMutableTreeNode("Leaf 2");

    // Build our tree model starting at the root node, and then make a JTree out
    // of it.
    treeModel = new DefaultTreeModel(root);
    tree = new JTree(treeModel);

    // Build the tree up from the nodes we created.
    treeModel.insertNodeInto(subroot, root, 0);
    // Or, more succinctly:
    subroot.add(leaf1);
    root.add(leaf2);

    // Display it.
    getContentPane( ).add(tree, BorderLayout.CENTER);
  }

  public static void main(String args[]) {
    TestTree tt = new TestTree( );
    tt.init( );
    tt.setVisible(true);
  }
}

public class Tree {
    private List<Tree> leaves = new LinkedList<Tree>();
    private Tree parent = null;
    private String data;

    public Tree(String data, Tree parent) {
        this.data = data;
        this.parent = parent;
    }
}

显然，您可以添加实用工具方法来添加/删除子元素。

在过去，我只是为此使用了一个嵌套映射。这是我今天用的，它很简单，但它符合我的需要。也许这能帮到另一个人。

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"));
    }
}

这个呢?

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;
  }
}

简单的例子:

public class ArbrePlaner {

public static void main(String[] args) {
    ArbrePlaner ll = new ArbrePlaner();
    
    ll.add(1,"A");
    ll.add(2,"B");
    ll.add(1,"C");
    ll.add(3,"D");
    ll.add(1,"Z");
    
    for(int i = 0; i < ll.size; i++){
    //  System.out.println(ll.isIdExist(i));
        System.out.println("-----------------");
        System.out.println(ll.getIdAt(i)+" :");
        linkedList lst = ll.getListDataById(ll.getIdAt(i));
        for(int j = 0; j < lst.size; j++){
            System.out.println(lst.getElementAt(j));
        }
    }
    
    
    
    
}

private int size;
private Noeud root;

public Noeud add(long Id, Object data){
    if(isIdExist(Id)){
        Noeud nd = getNoeudId(Id);
        nd.add(data);
        return nd;
    }else{
        Noeud nd = new Noeud(Id, data, this.root);
        this.root = nd;
        this.size++;
        return nd;
    }
}
 
 public Object getDataById(long Id, int x){
        Noeud thisNode = this.root;
        while(thisNode!=null){
            if(thisNode.getId() == Id){
                return thisNode.getLl().getElementAt(x);
            }
            thisNode = thisNode.getNextNoeud();
        }
        return null;
    }
 
 public long getIdAt(int x){
        if(size >= x){
            Noeud nd = this.root;
            for(int i = 0; i<x; i++)try {nd = nd.getNextNoeud();} catch (Exception e) {return -1;}
            return nd.getId();
        }
            return -1;
    }
 
 public linkedList getListDataById(long Id){
        Noeud thisNode = this.root;
        while(thisNode!=null){
            if(thisNode.getId() == Id){
                return thisNode.getLl();
            }
            thisNode = thisNode.getNextNoeud();
        }
        return null;
    }
 
public boolean deleteById(long id){
    Noeud thisNode = this.root;
    Noeud prevNode = null;
    
    while(thisNode != null){
        if(thisNode.getId() == id){
            prevNode.setNextNoeud(thisNode.getNextNoeud());
            this.setSize(this.getSize()-1);
            return true;
        }
        prevNode = thisNode;
        thisNode = thisNode.getNextNoeud();
    }
    return false;
}

 public boolean isIdExist(long Id){
        Noeud thisNode = this.root;
        while(thisNode!=null){
            if(thisNode.getId()== Id){
                return true;
            }
            thisNode = thisNode.getNextNoeud();
        }
        return false;
    }

 public boolean isDataExist(long Id, Object data){
     if(isIdExist(Id)){
         Noeud thisNode = this.root;
            while(thisNode!=null){
                if(thisNode.getId() == Id){
                    linkedList ll = thisNode.getLl();
                    long x = ll.hashCode();
                    long y = data.hashCode();
                    if(x==y) return true;
                }
                thisNode = thisNode.getNextNoeud();
            }
     }
     return false;
 }
 
 public Noeud getNoeudId(long Id){
        Noeud thisNode = this.root;
        while(thisNode!=null){
            if(thisNode.getId() == Id){
                return thisNode;
            }
            thisNode = thisNode.getNextNoeud();
        }
        return null;
    }

public ArbrePlaner() {
    this.root = root;
}

public ArbrePlaner(Noeud root) {
    this.root = root;
}

public ArbrePlaner(int size, Noeud root) {
    this.size = size;
    this.root = root;
}

public int getSize() {
    return size;
}

public void setSize(int size) {
    this.size = size;
}

public Noeud getRoot() {
    return root;
}

public void setRoot(Noeud root) {
    this.root = root;
}

private class Noeud{
    private long id;
    private Noeud nextNoeud;
    private linkedList Ll;
    
    public void add(Object data){
        Ll.add(data);
    }
    
    public Noeud(long id, Object data ,Noeud nextNoeud){
        this.id = id;
        this.nextNoeud = nextNoeud;
        Ll = new linkedList();
        Ll.add(data);
    }
    
    public long getId() {
        return id;
    }
    
    public Noeud(Object data){
        Ll.add(data);
    }
            
    public void setId(long id) {
        this.id = id;
    }
    public Noeud getNextNoeud() {
        return nextNoeud;
    }
    public void setNextNoeud(Noeud nextNoeud) {
        this.nextNoeud = nextNoeud;
    }
    public linkedList getLl() {
        return Ll;
    }
    public void setLl(linkedList ll) {
        Ll = ll;
    }
}
}