策略模式Strategy

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.strategy;

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

 AddStrategy.java

package org.fool.strategy;

public class AddStrategy implements Strategy {

	@Override
	public int calculate(int a, int b) {
		return a + b;
	}

}

 SubtractStrategy.java

package org.fool.strategy;

public class SubtractStrategy implements Strategy {

	@Override
	public int calculate(int a, int b) {
		return a - b;
	}

}

 MultiplyStrategy.java

package org.fool.strategy;

public class MultiplyStrategy implements Strategy {

	@Override
	public int calculate(int a, int b) {
		return a * b;
	}

}

 DivideStrategy.java

package org.fool.strategy;

public class DivideStrategy implements Strategy {

	@Override
	public int calculate(int a, int b) {
		return a / b;
	}

}

 Environment.java

package org.fool.strategy;

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);
	}

}

 Client.java

package org.fool.strategy;

public class Client {
	public static void main(String[] args) {
		Strategy addStrategy = new AddStrategy();

		Environment environment = new Environment(addStrategy);
		System.out.println(environment.calculate(3, 2));

		Strategy substractStrategy = new SubtractStrategy();
		environment.setStrategy(substractStrategy);
		System.out.println(environment.calculate(3, 2));

		Strategy multiplyStrategy = new MultiplyStrategy();
		environment.setStrategy(multiplyStrategy);
		System.out.println(environment.calculate(3, 2));

		Strategy devideStrategy = new DivideStrategy();
		environment.setStrategy(devideStrategy);
		System.out.println(environment.calculate(3, 2));
	}
}