定义

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。备忘录模式属于行为型模式。

实现

备忘录模式使用三个类 Memento、Originator 和 CareTaker。Memento 包含了要被恢复的对象的状态。Originator 创建并在 Memento 对象中存储状态。Caretaker 对象负责从 Memento 中恢复对象的状态。

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步骤 1创建 Memento 类。Memento.javapublic 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.javapublic 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.javaimport 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);   }}步骤 4使用 CareTaker 和 Originator 对象。MementoPatternDemo.javapublic 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 #4First saved State: State #2Second saved State: State #3