策略模式Strategy

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1. 定义

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。本模式使得算法可独立于使用它的客户而变化。

2.结构图

• Strategy：策略接口，用来约束一系列具体的策略算法。Context使用这个接口来调用具体的策略实现定义的算法。
• ConcreteStrategy：具体的策略实现，也就是具体的算法实现。
• Context：上下文，负责和具体的策略类交互。通常上下文会持有一个真正的策略实现，上下文还可以让具体的策略类来获取上下文的数据，甚至让具体的策略类来回调上下文的方法。

3. 本质

策略模式的本质：分离算法，选择实现。

4.Code Demo

Strategy.java

package org.fool.dp.strategy;

public interface Strategy {
    int calculate(int a, int b);
}

AddStrategy.java

package org.fool.dp.strategy;

public class AddStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a + b;
    }
}

SubtractStrategy.java

package org.fool.dp.strategy;

public class SubtractStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a - b;
    }
}

MultiplyStrategy.java

package org.fool.dp.strategy;

public class MultiplyStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a * b;
    }
}

DivideStrategy.java

package org.fool.dp.strategy;

public class DivideStrategy implements Strategy {
    @Override
    public int calculate(int a, int b) {
        return a / b;
    }
}

Context.java

package org.fool.dp.strategy;

public class Context {
    private Strategy strategy;

    public Context(Strategy strategy) {
        this.strategy = strategy;
    }

    public int calculate(int a, int b) {
        return strategy.calculate(a, b);
    }
}

Client.java

package org.fool.dp.strategy;

public class Client {
    public static void main(String[] args) {
        Strategy addStrategy = new AddStrategy();
        Context context = new Context(addStrategy);
        System.out.println(context.calculate(1, 2));

        Strategy subtractStrategy = new SubtractStrategy();
        context = new Context(subtractStrategy);
        System.out.println(context.calculate(1, 2));

        Strategy multiplyStrategy = new MultiplyStrategy();
        context = new Context(multiplyStrategy);
        System.out.println(context.calculate(1, 2));

        Strategy devideStrategy = new DivideStrategy();
        context = new Context(devideStrategy);
        System.out.println(context.calculate(1, 2));
    }
}