3. 行为型模式 (1) 策略模式

3. 行为型模式 (1) 策略模式

在策略模式（Strategy Pattern）中，一个类的行为或其算法可以在运行时更改。这种类型的设计模式属于行为型模式。

在策略模式中，我们创建表示各种策略的对象和一个行为随着策略对象改变而改变的 context 对象。策略对象改变 context 对象的执行算法。

优点：
1. 算法可以自由切换。
2. 避免使用多重条件判断。
3. 扩展性良好。

缺点：
1. 策略类会增多。
2. 所有策略类都需要对外暴露。

1. 策略模式

package com.andrew.pattern0301.strategy.model01;
public interface Strategy {
    public int doOperation(int num1, int num2);
}

package com.andrew.pattern0301.strategy.model01;
public class OperationAdd implements Strategy {
    @Override
    public int doOperation(int num1, int num2) {
        return num1 + num2;
    }
}

package com.andrew.pattern0301.strategy.model01;
public class OperationMultiply implements Strategy {
    @Override
    public int doOperation(int num1, int num2) {
        return num1 - num2;
    }
}

package com.andrew.pattern0301.strategy.model01;
public class OperationSubstract implements Strategy {
    @Override
    public int doOperation(int num1, int num2) {
        return num1 * num2;
    }
}

package com.andrew.pattern0301.strategy.model01;
public class Context {
    private Strategy strategy;
    public Context(Strategy strategy) {
        this.strategy = strategy;
    }
    public int executeStrategy(int num1, int num2) {
        return strategy.doOperation(num1, num2);
    }
}

package com.andrew.pattern0301.strategy.model01;
/**
 * 1. 策略模式
 * 
 * @author andrew
 * @date 2018/08/01
 */
public class Client {
    public static void main(String[] args) {
        Context context = new Context(new OperationAdd());
        System.out.println("10 + 5 = " + context.executeStrategy(10, 5));
        context = new Context(new OperationSubstract());
        System.out.println("10 - 5 = " + context.executeStrategy(10, 5));
        context = new Context(new OperationMultiply());
        System.out.println("10 * 5 = " + context.executeStrategy(10, 5));
    }
}
运行结果：
10 + 5 = 15
10 - 5 = 50
10 * 5 = 5

2. 策略模式

package com.andrew.pattern0301.strategy.model02;
public interface Strategy {
    public int calculate(int a, int b);
}

package com.andrew.pattern0301.strategy.model02;
public class AddStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a + b;
    }
}

package com.andrew.pattern0301.strategy.model02;
public class DivideStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a / b;
    }
}

package com.andrew.pattern0301.strategy.model02;
public class MultiplyStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a * b;
    }
}

package com.andrew.pattern0301.strategy.model02;
public class SubtractStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a - b;
    }
}

package com.andrew.pattern0301.strategy.model02;
public class Environment {
    private Strategy strategy;
    public Environment(Strategy strategy) {
        this.strategy = strategy;
    }
    public Strategy getStrategy() {
        return strategy;
    }
    public void setStrategy(Strategy strategy) {
        this.strategy = strategy;
    }
    public int calculate(int a ,int b) {
        return strategy.calculate(a , b);
    }
}

package com.andrew.pattern0301.strategy.model02;
/**
 * 2. 策略模式
 * 
 * @author andrew
 * @date 2018/08/01
 */
public class Client {
    public static void main(String[] args) {
        AddStrategy addStrategy = new AddStrategy();
        Environment environment = new Environment(addStrategy);
        System.out.println(environment.calculate(3, 4));
        SubtractStrategy subtractStrategy = new SubtractStrategy();
        environment.setStrategy(subtractStrategy);
        System.out.println(environment.calculate(3, 4));
        MultiplyStrategy multiplyStrategy = new MultiplyStrategy();
        environment.setStrategy(multiplyStrategy);
        System.out.println(environment.calculate(3, 4));
        DivideStrategy divideStrategy = new DivideStrategy();
        environment.setStrategy(divideStrategy);
        System.out.println(environment.calculate(3, 4));
    }
}
运行结果：
7
-1
12
0