《一天聊一个设计模式》备忘录

备忘录（Memento）

Intent

在不违反封装的情况下获得对象的内部状态，从而在需要时可以将对象恢复到最初状态。

Class Diagram

Originator：原始对象
Caretaker：负责保存好备忘录
Memento：备忘录，存储原始对象的的状态。备忘录实际上有两个接口，一个是提供给 Caretaker 的窄接口：它只能将备忘录传递给其它对象；一个是提供给 Originator 的宽接口，允许它访问到先前状态所需的所有数据。理想情况是只允许 Originator 访问本备忘录的内部状态。

Implementation

以下实现了一个简单计算器程序，可以输入两个值，然后计算这两个值的和。备忘录模式允许将这两个值存储起来，然后在某个时刻用存储的状态进行恢复。

实现参考：Memento Pattern - Calculator Example - Java Sourcecode

/*** Originator Interface*/
public interface Calculator {// Create MementoPreviousCalculationToCareTaker backupLastCalculation();// setMementovoid restorePreviousCalculation(PreviousCalculationToCareTaker memento);int getCalculationResult();void setFirstNumber(int firstNumber);void setSecondNumber(int secondNumber);
}

/*** Originator Implementation*/
public class CalculatorImp implements Calculator {private int firstNumber;private int secondNumber;@Overridepublic PreviousCalculationToCareTaker backupLastCalculation() {// create a memento object used for restoring two numbersreturn new PreviousCalculationImp(firstNumber, secondNumber);}@Overridepublic void restorePreviousCalculation(PreviousCalculationToCareTaker memento) {this.firstNumber = ((PreviousCalculationToOriginator) memento).getFirstNumber();this.secondNumber = ((PreviousCalculationToOriginator) memento).getSecondNumber();}@Overridepublic int getCalculationResult() {// result is adding two numbersreturn firstNumber + secondNumber;}@Overridepublic void setFirstNumber(int firstNumber) {this.firstNumber = firstNumber;}@Overridepublic void setSecondNumber(int secondNumber) {this.secondNumber = secondNumber;}
}

/*** Memento Interface to Originator** This interface allows the originator to restore its state*/
public interface PreviousCalculationToOriginator {int getFirstNumber();int getSecondNumber();
}

/***  Memento interface to CalculatorOperator (Caretaker)*/
public interface PreviousCalculationToCareTaker {// no operations permitted for the caretaker
}

/*** Memento Object Implementation* <p>* Note that this object implements both interfaces to Originator and CareTaker*/
public class PreviousCalculationImp implements PreviousCalculationToCareTaker,PreviousCalculationToOriginator {private int firstNumber;private int secondNumber;public PreviousCalculationImp(int firstNumber, int secondNumber) {this.firstNumber = firstNumber;this.secondNumber = secondNumber;}@Overridepublic int getFirstNumber() {return firstNumber;}@Overridepublic int getSecondNumber() {return secondNumber;}
}

/*** CareTaker object*/
public class Client {public static void main(String[] args) {// program startsCalculator calculator = new CalculatorImp();// assume user enters two numberscalculator.setFirstNumber(10);calculator.setSecondNumber(100);// find resultSystem.out.println(calculator.getCalculationResult());// Store result of this calculation in case of errorPreviousCalculationToCareTaker memento = calculator.backupLastCalculation();// user enters a numbercalculator.setFirstNumber(17);// user enters a wrong second number and calculates resultcalculator.setSecondNumber(-290);// calculate resultSystem.out.println(calculator.getCalculationResult());// user hits CTRL + Z to undo last operation and see last resultcalculator.restorePreviousCalculation(memento);// result restoredSystem.out.println(calculator.getCalculationResult());}
}