策略模式 (Strategy Pattern)

模式名称：策略模式（Strategy Pattern）

问题：希望定义一系列的算法，把它们一个个封装起来，并且使它们可相互替换。而直接使用if...else难以维护，且违反开闭原则。

解决方案：将这些算法封装成一个一个的类，并实现同一个接口，能任意进行替换。

效果：符合开闭原则，可自由切换策略。

public class Main {
    
    public static void main(String[] args) {
        // 可以自由切换策略
        // 也可以用setStrategy方法来变换策略
        Context context = new Context(new OperationAdd());    
        System.out.println("10 + 5 = " + context.executeStrategy(10, 5));
   
        context = new Context(new OperationSubtract());      
        System.out.println("10 - 5 = " + context.executeStrategy(10, 5));
   
        context = new Context(new OperationMultiply());    
        System.out.println("10 * 5 = " + context.executeStrategy(10, 5));
    }
}

class Context {
    
    private Strategy strategy;
  
    public Context(Strategy strategy){
        this.strategy = strategy;
    }
  
    public int executeStrategy(int num1, int num2){
        return strategy.doOperation(num1, num2);
    }
}

interface Strategy {
    public int doOperation(int num1, int num2);
}

class OperationAdd implements Strategy {
    
    @Override
    public int doOperation(int num1, int num2) {
        return num1 + num2;
    }
}
class OperationSubtract implements Strategy {
    
    @Override
    public int doOperation(int num1, int num2) {
        return num1 - num2;
    }
}
class OperationMultiply implements Strategy {
    
    @Override
    public int doOperation(int num1, int num2) {
        return num1 * num2;
    }
}