A detailed introduction to the Composite pattern in Java

Class diagram

##

/**
 * 树	整体
 * 
 * @author stone
 * 
 */
public class Tree {
	private TreeNode root; //根节点

	public Tree(String name) {
		this.root = new TreeNode(name);
	}
	
	public TreeNode getRoot() {
		return root;
	}
}

/**
 * 树节点 部份
 * 也可以自身代表树：一堆节点组成了一颗树
 * 
 * @author stone
 * 
 */
public class TreeNode {

	private String name;
	private TreeNode parent;
	private List<TreeNode> children;
	
	public TreeNode(String name) {
		this.name = name;
		this.children = new ArrayList<TreeNode>();
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public void setParent(TreeNode parent) {
		this.parent = parent;
	}

	public TreeNode getParent() {
		return parent;
	}
	
	public List<TreeNode> getChildren() {
		return children;
	}
	
	public void add(TreeNode node) {
		this.children.add(node);
	}
	
	public void remove(TreeNode node) {
		this.children.remove(node);
	}
}

/*
 * 组合(Composite)模式  又叫做部分-整体模式
 * 它使我们层级、树形结构的问题中，模糊了简单元素和复杂元素的概念，客户程序可以像处理简单元素一样来处理复杂元素,从而使得客户程序与复杂元素的内部结构解耦
 * 以下情况下适用Composite模式：
		1．你想表示对象的部分-整体层次结构
		2．你希望用户忽略组合对象与单个对象的不同，用户将统一地使用组合结构中的所有对象。
 */
public class Test {
	public static void main(String[] args) {
//		Tree treeA = new Tree("A");
//		treeA.getRoot().add(new TreeNode("B"));
//		treeA.getRoot().add(new TreeNode("C"));
//		TreeNode treeNodeD = new TreeNode("D");
//		treeNodeD.add(new TreeNode("D1"));
//		treeNodeD.add(new TreeNode("D2"));
//		treeA.getRoot().add(treeNodeD);
//		print(treeA.getRoot());
		/*
		 * 上面使用了Tree对象，
		 * 下面只使用TreeNode对象，符合组合模式的定义，既代表部分也代表整体
		 */
		TreeNode treeA = new TreeNode("A");
		treeA.add(new TreeNode("B"));
		treeA.add(new TreeNode("C"));
		TreeNode treeNodeD = new TreeNode("D");
		treeNodeD.add(new TreeNode("D1"));
		treeNodeD.add(new TreeNode("D2"));
		treeA.add(treeNodeD);
		print(treeA);
		/*
		 * 其他示例：文件系统{目录、文件}, 类似这种可用递归遍历的结构，
		 * 用一个对象就能表示部分与整体，都可以用组合模式
		 */

	}
	
	public static void print(TreeNode root) {
		if (root == null) return;
		LinkedList<TreeNode> linkedList = new LinkedList<TreeNode>();  
		linkedList.add(root);
		while (!linkedList.isEmpty()) {
			TreeNode node = linkedList.removeFirst();
			System.out.println(node.getName());
			List<TreeNode> children = node.getChildren();
			for (int i = 0; i < children.size(); i++) {
				TreeNode next = children.get(i);
				List<TreeNode> children2 = next.getChildren();
				if (!children2.isEmpty()) {
					linkedList.add(next);
				} else {
					System.out.println(next.getName());
				}
			}
		}
	}
}

Print

A
B
C
D
D1
D2

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