《一天聊一个设计模式》备忘录

r囧r小猫 2022-11-07 04:26 128阅读 0赞

## 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 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

[Memento Pattern - Calculator Example - Java Sourcecode]: https://www.oodesign.com/memento-pattern-calculator-example-java-sourcecode.html