备忘录模式(Memento Pattern)

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保存一个对象的某个状态，以便在适当的时候恢复对象的先前状态。编辑器中的redo和undo操作一样，所实现的操作就是备忘录模式。备忘录模式归属于行为型模式。

在下面的实例中，类Memento定义了备忘录的基本信息，类Originator创建我存储备忘录类对象信息，类Caretaker对象负责存储来自备忘录的对象状态。下面总共创建了3个类，Memento, Originator 和 CareTaker. UML 图如下所示：

备忘录类Memento.java

public class Memento {   private String state;    public Memento(String state){      this.state = state;   }    public String getState(){      return state;   }      }

原始保存类Originator.java

public class Originator {   private String state;    public void setState(String state){      this.state = state;   }    public String getState(){      return state;   }    public Memento saveStateToMemento(){      return new Memento(state);   }    public void getStateFromMemento(Memento memento){      state = memento.getState();   }}

检索操作类CareTaker.java

import java.util.ArrayList;import java.util.List; public class CareTaker {   private List<Memento> mementoList = new ArrayList<Memento>();    public void add(Memento state){      mementoList.add(state);   }    public Memento get(int index){      return mementoList.get(index);   }}

测试类MementoPatternDemo.java

public class MementoPatternDemo {   public static void main(String[] args) {      Originator originator = new Originator();      CareTaker careTaker = new CareTaker();      originator.setState("State #1");      originator.setState("State #2");      careTaker.add(originator.saveStateToMemento());      originator.setState("State #3");      careTaker.add(originator.saveStateToMemento());      originator.setState("State #4");       System.out.println("Current State: " + originator.getState());                        originator.getStateFromMemento(careTaker.get(0));      System.out.println("First saved State: " + originator.getState());      originator.getStateFromMemento(careTaker.get(1));      System.out.println("Second saved State: " + originator.getState());   }}

输出结果：

Current State: State #4First saved State: State #2Second saved State: State #3

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