备忘录模式是什么？

在不破坏封装性的前提下，捕获一个对的的内部状态，并在该对象之外保持这个状态。这样以后就可将该对象恢复到原先保持的状态

备忘录模式解决什么问题？

如游戏打Boss失败时回档，定义一个游戏角色

public class GameRole {private int vitality;private int attack;private int defense;public int getVitality() {return vitality;}public void setVitality(int vitality) {this.vitality = vitality;}public int getAttack() {return attack;}public void setAttack(int attack) {this.attack = attack;}public int getDefense() {return defense;}public void setDefense(int defense) {this.defense = defense;}public void displayState() {System.out.println("角色当前状态：");System.out.println("体力：" + vitality);System.out.println("攻击力：" + attack);System.out.println("防御力：" + defense);}public void getInitState() {vitality = 100;attack = 100;defense = 100;}public void fight() {vitality = 0;attack = 0;defense = 0;}
}

在挑战Boss前，复制一份当前的状态

GameRole role = new GameRole();
role.getInitState();
role.displayState();GameRole backup = new GameRole();
backup.setVitality(role.getVitality());
backup.setAttack(role.getAttack());
backup.setDefense(role.getDefense());

当挑战Boss失败后，通过此前的复制还原状态

role.fight();
role.displayState();role.setVitality(backup.getVitality());
role.setAttack(backup.getAttack());
role.setDefense(backup.getDefense());
role.displayState();

• problem1：需要对属性一个个进行备份
• problem2：当属性增加或修改时，需要对代码修改

备忘录模式实现

创建备忘录，添加要保存的数据

public class RoleStateMemento {private int vitality;private int attack;private int defense;public RoleStateMemento(int vitality, int attack, int defense) {this.vitality = vitality;this.attack = attack;this.defense = defense;}public int getVitality() {return vitality;}public void setVitality(int vitality) {this.vitality = vitality;}public int getAttack() {return attack;}public void setAttack(int attack) {this.attack = attack;}public int getDefense() {return defense;}public void setDefense(int defense) {this.defense = defense;}
}

修改GameRole类，添加备份/还原方法

public class GameRole {private int vitality;private int attack;private int defense;public int getVitality() {return vitality;}public void setVitality(int vitality) {this.vitality = vitality;}public int getAttack() {return attack;}public void setAttack(int attack) {this.attack = attack;}public int getDefense() {return defense;}public void setDefense(int defense) {this.defense = defense;}public void displayState() {System.out.println("角色当前状态：");System.out.println("体力：" + vitality);System.out.println("攻击力：" + attack);System.out.println("防御力：" + defense);}public void getInitState() {vitality = 100;attack = 100;defense = 100;}public void fight() {vitality = 0;attack = 0;defense = 0;}public RoleStateMemento saveState() {return new RoleStateMemento(vitality, attack, defense);}public void recoveryState(RoleStateMemento memento) {vitality = memento.getVitality();attack = memento.getAttack();defense = memento.getDefense();}
}

创建它们之间的管理类

public class RoleStateCaretaker {private RoleStateMemento mMemento;public RoleStateMemento getMemento() {return mMemento;}public void setMemento(RoleStateMemento memento) {mMemento = memento;}
}

备份/还原代码如下

GameRole role = new GameRole();
role.getInitState();
role.displayState();RoleStateCaretaker stateCaretaker = new RoleStateCaretaker();
stateCaretaker.setMemento(role.saveState());role.fight();
role.displayState();role.recoveryState(stateCaretaker.getMemento());
role.displayState();