策略模式 Strategy

策略模式（Strategy）：它定义了一系列的算法，并将每一个算法封装起来，而且使它们还可以相互替换。策略模式让算法的变化不会影响到使用算法的客户。（原文：The Strategy Pattern defines a family of algorithms,encapsulates each one,and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.）

UML:

Strategy - defines an interface common to all supported algorithms. Context uses this interface to call the algorithm defined by a ConcreteStrategy.

ConcreteStrategy - each concrete strategy implements an algorithm.

Context

• contains a reference to a strategy object.
• may define an interface that lets strategy accessing its data.

The Context objects contains a reference to the ConcreteStrategy that should be used. When an operation is required then the algorithm is run from the strategy object. The Context is not aware of the strategy implementation. If necessary, addition objects can be defined to pass data from context object to strategy.

The context object receives requests from the client and delegates them to the strategy object. Usually the ConcreteStartegy is created by the client and passed to the context. From this point the clients interacts only with the context.

例子：

public interface IBehaviour {
	public int moveCommand();
}

public class AgressiveBehaviour implements IBehaviour{
	public int moveCommand()
	{
		System.out.println("\tAgressive Behaviour: if find another robot attack it");
		return 1;
	}
}

public class DefensiveBehaviour implements IBehaviour{
	public int moveCommand()
	{
		System.out.println("\tDefensive Behaviour: if find another robot run from it");
		return -1;
	}
}

public class NormalBehaviour implements IBehaviour{
	public int moveCommand()
	{
		System.out.println("\tNormal Behaviour: if find another robot ignore it");
		return 0;
	}
}

public class Robot {
	IBehaviour behaviour;
	String name;

	public Robot(String name)
	{
		this.name = name;
	}

	public void setBehaviour(IBehaviour behaviour)
	{
		this.behaviour = behaviour;
	}

	public IBehaviour getBehaviour()
	{
		return behaviour;
	}

	public void move()
	{
		System.out.println(this.name + ": Based on current position" +
					 "the behaviour object decide the next move:");
		int command = behaviour.moveCommand();
		// ... send the command to mechanisms
		System.out.println("\tThe result returned by behaviour object " +
					"is sent to the movement mechanisms " + 
					" for the robot '"  + this.name + "'");
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
}


public class Main {

	public static void main(String[] args) {

		Robot r1 = new Robot("Big Robot");
		Robot r2 = new Robot("George v.2.1");
		Robot r3 = new Robot("R2");

		r1.setBehaviour(new AgressiveBehaviour());
		r2.setBehaviour(new DefensiveBehaviour());
		r3.setBehaviour(new NormalBehaviour());

		r1.move();
		r2.move();
		r3.move();

		System.out.println("\r\nNew behaviours: " +
				"\r\n\t'Big Robot' gets really scared" +
				"\r\n\t, 'George v.2.1' becomes really mad because" +
				"it's always attacked by other robots" +
				"\r\n\t and R2 keeps its calm\r\n");

		r1.setBehaviour(new DefensiveBehaviour());
		r2.setBehaviour(new AgressiveBehaviour());

		r1.move();
		r2.move();
		r3.move();
	}
}