设计模式之备忘录模式

主要用来保存对象状态，并提供恢复机制。

import java.util.Stack;

public class Test {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Caretaker ct = new Caretaker();
		
		Originator originator = new Originator();
		System.out.println("操作1");
		originator.setState("操作1");
		ct.addMemento(originator.createMemento());  
		// 不要写成ct.addMemento(new Memento(originator));， 即不能让客户端感知道备忘录对象的存在
		System.out.println("操作2");
		originator.setState("操作2");
		ct.addMemento(originator.createMemento());
		System.out.println("操作3");
		originator.setState("操作3");
		ct.addMemento(originator.createMemento());
		System.out.println("操作4");
		originator.setState("操作4");
		ct.addMemento(originator.createMemento());
		
		originator.restoreMemento(ct.getMemento());
		System.out.println("恢复到：" + originator.getState());
		originator.restoreMemento(ct.getMemento());
		System.out.println("恢复到：" + originator.getState());
		originator.restoreMemento(ct.getMemento());
		System.out.println("恢复到：" + originator.getState());
		originator.restoreMemento(ct.getMemento());
		System.out.println("恢复到：" + originator.getState());
		originator.restoreMemento(ct.getMemento());
		System.out.println("恢复到：" + originator.getState());
		originator.restoreMemento(ct.getMemento());
		System.out.println("恢复到：" + originator.getState());
	}
}

// 被备份的对象
class Originator {
	private String state;
	
	public Originator () {
		
	}
	
	public Memento createMemento () {
		return new Memento(this);
	}
	
	public void restoreMemento(Memento memento) {
		if (memento == null) {
			System.out.println("已恢复到尽头。。。");
			return;
		}
		this.state = memento.getState();
	}

	public String getState() {
		return state;
	}

	public void setState(String state) {
		this.state = state;
	}
}

// 备忘录
class Memento {
	private String state;
	
	// 把对象的状态保存到备忘录
	public Memento (Originator originator) {
		setState(originator.getState());
	}
	
	public String getState() {
		return state;
	}
	public void setState(String state) {
		this.state = state;
	}
}

// 备忘录容器
class Caretaker {
	private static final int MAX_SIZE = 1000;
	
	private Stack<Memento> stack = new Stack<Memento>();

	public void addMemento (Memento memento) {
		if (stack.size() > MAX_SIZE) {
			return;  // No more memento
		}
		stack.push(memento);
	}
	
	public Memento getMemento () {
		if (stack.isEmpty()) {
			return null;
		}
		return stack.pop();
	}
	
}

输出：

操作1
操作2
操作3
操作4
恢复到：操作4
恢复到：操作3
恢复到：操作2
恢复到：操作1
已恢复到尽头。。。
恢复到：操作1          // 最后一次恢复的状态
已恢复到尽头。。。
恢复到：操作1