一、概念

备忘录模式（Memento Pattern）：也叫快照（Snapshot）模式，在不违背封装原则的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态，以便之后恢复对象为先前的状态。通俗点就是用于保存某个对象的状态，以便在适当的时候恢复对象。简单的应用比如：游戏存档、浏览器中的后退、word中的撤销。

二、实现

这里举一个word中添加撤销的例子。
1、Word

public class Word {StringBuilder content = new StringBuilder();public String getContent() {return content.toString();}public void append(String input) {content.append(input);}public WordMemento createWordMemento() {return new WordMemento(content.toString());}public void restoreWordMemento(WordMemento wordMemento) {content.replace(0, content.length(), wordMemento.getContent());}
}

2、WordMemento表示word操作过程状态，该数据一旦创建不能留修改入口。

public class WordMemento {private String content;public WordMemento(String content) {this.content = content;}public String getContent() {return content;}
}

3、WordMementoHolder状态控制类

public class WordMementoHolder {private Stack<WordMemento> stack = new Stack<>();private static class WordMementoHolderSingleton {private static final WordMementoHolder INSTANCE = new WordMementoHolder();}public static WordMementoHolder getInstance() {return WordMementoHolderSingleton.INSTANCE;}public void pushWordMemento(WordMemento wordMemento) {stack.push(wordMemento);}public WordMemento popWordMemento() {return stack.pop();}public WordMemento getStackTopMemento(){return stack.peek();}
}

4、测试类

public class Client {public static void main(String[] args) {WordMementoHolder wordMementoHolder = WordMementoHolder.getInstance();Word word = new Word();String str1 = "hello";String str2 = "123";String str3 = "java";//添加第一个word.append(str1);wordMementoHolder.pushWordMemento(word.createWordMemento());System.out.println(word.getContent());//添加第二个word.append(str2);wordMementoHolder.pushWordMemento(word.createWordMemento());System.out.println(word.getContent());//添加第三个word.append(str3);wordMementoHolder.pushWordMemento(word.createWordMemento());System.out.println(word.getContent());//撤销第三个wordMementoHolder.popWordMemento();word.restoreWordMemento(wordMementoHolder.getStackTopMemento());System.out.println(word.getContent());}
}

5、运行结果