行为型 - 备忘录(Memento)

张启忻大约 2 分钟

行为型 - 备忘录(Memento)

提示

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。

[行为型 - 备忘录(Memento)](#行为型 - 备忘录(memento))

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。

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 ](https://www.oodesign.com/memento-pattern-calculator-example-java- sourcecode.html)


    /**
     * 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