标题：Java备忘录模式

备忘录模式(Memento Pattern)用来恢复一个对象的状态到以前的状态。 备忘录模式下属于行为模式的类别。

实现实例

在这个示例中，备忘录模式(Memento Pattern)使用三个操作者类。 Memento类中包含要恢复的对象的状态。 创建者在Memento对象中创建和存储状态，Caretaker对象负责从Memento中恢复对象状态。在这个示例中，创建了以下几个类：Memento，Originator和CareTaker。

MementoPatternDemo这是一个演示类，它将使用CareTaker和Originator对象来显示对象状态的恢复。

备忘录模式(Memento Pattern)示例的结构如下图所示 -

第1步

创建一个Memento类，其代码如下-
Memento.java

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

public class Memento {
   private String state;

   public Memento(String state){
      this.state = state;
   }

   public String getState(){
      return state;
   }    
}
Java

第2步

创建一个Originator类，其代码如下-
Originator.java

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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();
   }
}
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();
   }
}
Java

第3步

创建一个CareTaker类，其代码如下-
CareTaker.java

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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);
   }
}
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);
   }
}
Java

第4步

创建一个类-MementoPatternDemo并使用CareTaker和Originator对象。
MementoPatternDemo.java

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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());
   }
}
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());
   }
}
Java

第5步

验证输出，执行上面的代码得到以下结果 -

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

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