設計模式 - 備忘錄模式

備忘錄模式用於將物件的狀體恢復到之前的狀體。備忘錄模式屬於行為模式類別。

實現

備忘錄模式使用三個角色類。備忘錄包含要恢復的物件的狀體。發起人建立並存儲備忘錄中的狀體，而管理者負責從備忘錄中恢復物件狀體。我們建立了Memento、Originator和CareTaker類。

我們的演示類MementoPatternDemo將使用CareTaker和Originator物件來展示物件狀體的恢復。

步驟1

建立Memento類。

Memento.java

public class Memento {
   private String state;

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

   public String getState(){
      return state;
   }	
}

步驟2

建立Originator類

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();
   }
}

步驟3

建立CareTaker類

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);
   }
}

步驟4

使用CareTaker和Originator物件。

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());
   }
}

步驟5

驗證輸出。

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