备忘录模式（Memento）

目的

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。

类图

• 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他对象；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。

实现

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 Memento
    PreviousCalculationToCareTaker backupLastCalculation();
    // setMemento
    void 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;
    @Override
    public PreviousCalculationToCareTaker backupLastCalculation() {
        // create a memento object used for restoring two numbers
        return new PreviousCalculationImp(firstNumber, secondNumber);
    }
    @Override
    public void restorePreviousCalculation(PreviousCalculationToCareTaker memento) {
        this.firstNumber = ((PreviousCalculationToOriginator) memento).getFirstNumber();
        this.secondNumber = ((PreviousCalculationToOriginator) memento).getSecondNumber();
    }
    @Override
    public int getCalculationResult() {
        // result is adding two numbers
        return firstNumber + secondNumber;
    }
    @Override
    public void setFirstNumber(int firstNumber) {
        this.firstNumber = firstNumber;
    }
    @Override
    public 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;
    }
    @Override
    public int getFirstNumber() {
        return firstNumber;
    }
    @Override
    public int getSecondNumber() {
        return secondNumber;
    }
}
/**
 * CareTaker object
 */
public class Client {
    public static void main(String[] args) {
        // program starts
        Calculator calculator = new CalculatorImp();
        // assume user enters two numbers
        calculator.setFirstNumber(10);
        calculator.setSecondNumber(100);
        // find result
        System.out.println(calculator.getCalculationResult());
        // Store result of this calculation in case of error
        PreviousCalculationToCareTaker memento = calculator.backupLastCalculation();
        // user enters a number
        calculator.setFirstNumber(17);
        // user enters a wrong second number and calculates result
        calculator.setSecondNumber(-290);
        // calculate result
        System.out.println(calculator.getCalculationResult());
        // user hits CTRL + Z to undo last operation and see last result
        calculator.restorePreviousCalculation(memento);
        // result restored
        System.out.println(calculator.getCalculationResult());
    }
}
运行结果
110
-273
110

JDK中的体现

• java.io.Serializable