java中的特定树数据结构

Question

I'm trying to implement a specific tree data structure in Java; I don't know exactly what type of tree is this. Here's an example of what I'm trying to do:

             -----------------------------
            | Board                       |
            |
            | Node1  Node2  Node3  Node4  |
            |   _      _      _      _
            |  |_|    |_|    |_|    |_|   |
             ---+------+------+------+----
                |      |      |      |
               /       |      |       \
 --------------       ---    ---     -----------
| Board        |       -      -     | Board     |
|                                   |           
| Node1  Node2 |                    | Node1 ... |
|   _      _                        |   _       
|  |_|    |_|  |                    |  |_|  ... |
 ---+------+---                      ---+------+ 
   /       |                            |
   .       .                            .
   .       .                            .
   .       .                            .

So I created two classes: Board and Node .

• Each Board consists of an ArrayList of Nodes :

   public class Board { ArrayList<Node> mContent; Board() { mContent = new ArrayList<Node>(); } Board(Board pBoard) { mContent = new ArrayList<Node>(pBoard.mContent); } void add(Node pNode) { mContent.add(pNode); } void add(String pString, Board pBoard) { Node tNode = new Node(pString, pBoard); mContent.add(tNode); } } 

• Each Node consists of a String and a reference to another Board :

   public class Node { String mLabel; Board mBoard; Node(){ mLabel = new String(); mBoard = null; } Node(String pLabel, Board pBoard){ mLabel = new String(pLabel); mBoard = new Board(Board); } void setBoard(Board pBoard){ mBoard = pBoard; } } 

My questions are:

• On the add(Node pNode) method, do I need to create a new Node and then add it to the ArrayList ?

• On the setBoard(...) method, do I need to create a new Board and then pass it to mBoard , or just doing mBoard = pBoard is correct?

• How do I indicate a leaf Node ? I tried to initialize it with null , but I got a NullPointerException

Thank you!

Answer 1

To answer your question:

• The Node passed as pNode should be used; you don't need to create a new one.
• The Board passed as pBoard should be used; you don't need to create a new one.
• A leaf node has a null mBoard . Alternatively, it could have an mBoard that has an empty mContent array. The choice depends on information you have not posted about how you want your trees to bottom out.

You might consider collapsing your structure. It's not clear why you don't just have a single Node class (with an ArrayList<Node> mContent field in place of the mBoard field).