行为型 - 备忘录(Memento)

备忘录模式(Memento pattern): 当你需要让对象返回之前的状态时(例如, 你的用户请求"撤销"), 你使用备忘录模式。@pdai

• 行为型 - 备忘录(Memento)

• 意图

• 类图

• 实现

• JDK

意图

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
• Menento: 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。

867e93eb-3161-4f39-b2d2-c0cd3788e194.png

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 在新窗口打开

    /** * 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());
        }
    }

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JDK

• java.io.Serializable