备忘录模式(Memento Pattern)

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< http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHA+PGVtPjwvZW0+PC9wPgo8cHJlIGNsYXNzPQ=="brush: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
  
    mementoList = new ArrayList
   
    (); 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 #4
First saved State: State #2
Second saved State: State #3