设计模式 - 备忘录模式

备忘录模式用于将对象的状体恢复到之前的状体。备忘录模式属于行为模式类别。

实现

备忘录模式使用三个角色类。备忘录包含要恢复的对象的状体。发起人创建并存储备忘录中的状体，而管理者负责从备忘录中恢复对象状体。我们创建了Memento、Originator和CareTaker类。

我们的演示类MementoPatternDemo将使用CareTaker和Originator对象来展示对象状体的恢复。

步骤1

创建Memento类。

Memento.java

public class Memento {
   private String state;

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

   public String getState(){
      return state;
   }	
}

步骤2

创建Originator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();
   }
}

步骤3

创建CareTaker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);
   }
}

步骤4

使用CareTaker和Originator对象。

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());
   }
}

步骤5

验证输出。

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