设计模式之备忘录模式

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概述：在不破坏封装性的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态。这样以后就可将该对象恢复到原先保存的状态

角色：

Memento 备忘录存储原发器对象的内部状态
Originator 原发器创建一个备忘录,用以记录当前时刻它的内部状态。使用备忘录恢复内部状态
Caretaker 负责保存好备忘录。不能对备忘录的内容进行操作或检查

类图：
这里写图片描述

实现：

public class Originator {    private String state;    public Originator(String state) {        this.state = state;    }    //保存    public Memento createMemento() {        return new Memento(state);    }    //返回    public void history(Memento memento) {        state = memento.getState();    }    public String getState() {        return state;    }    public void setState(String state) {        this.state = state;    }    @Override    public String toString() {        System.out.println("Originator{" +                "state='" + state + '\'' +                '}');        return "Originator{" +                "state='" + state + '\'' +                '}';    }}

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

public class Caretaker {    private ArrayList<Memento> mementoList ;    public Memento getMemento(int i){        if(null!=mementoList){            return mementoList.get(i);        }        return null;    }    public void setMemento(Memento memento){        if(null == mementoList){            mementoList = new ArrayList<>();        }        mementoList.add(memento);    }}

public class testMemento {    //步数标志    private static int index = 0;    public static void main(String[] args) {        Caretaker caretaker = new Caretaker();        Originator originator = new Originator("1");        originator.toString();        caretaker.setMemento(originator.createMemento());        //每次备份，index+1        index++;        originator.setState("2");        originator.toString();        caretaker.setMemento(originator.createMemento());        index++;        originator.setState("3");        originator.toString();        originator.history(caretaker.getMemento(index-1));        //每次撤销，index-1        index--;        originator.toString();        originator.history(caretaker.getMemento(index-1));        index--;        originator.toString();    }}

打印：
Originator{state=’1’}
Originator{state=’2’}
Originator{state=’3’}
Originator{state=’2’}
Originator{state=’1’}

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