策略模式 (Strategy Pattern)

可以将一系列算法封装起来，可以进行自由切换，而不是 if...else 的复杂和难以维护。

也可以用 setStrategy 的方法来变换策略。

public class Main {
    
    public static void main(String[] args) {
        // 可以自由切换策略
        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;
    }
}