java 开发模式之十二 : 状态模式

原理或定义

当一个对象的内在状态改变时允许改变其行为，这个对象看起来像是改变了其类。状态模式中的行为是由状态来决定的，不同的状态对应了不同的行为。

结构

环境类Context: 它定义了客户程序需要的接口并维护一个具体状态角色的实例，将与状态相关的操作委托给当前的Concrete State对象来处理；
抽象状态类State: 定义一个接口以封装与Context的一个特定状态相关的行为；
具体状态类ConcreteState: 每一子类实现一个与Context的一个状态相关的行为；

类图

案例与代码

本模式使用一个糖果机项目来做示例

智能糖果机，用Java软件控制糖果机：
待机
投入一元硬币
转动把手
滑落一颗糖果
待机（根据机器内糖果库存情况，是否提示售罄）

传统实现：

public class CandyMachine {

	final static int SoldOutState = 0;
	final static int OnReadyState = 1;
	final static int HasCoin = 2;
	final static int SoldState = 3;

	private int state = SoldOutState;
	private int count = 0;

	public CandyMachine(int count) {
		this.count = count;
		if (count > 0) {
			state = OnReadyState;
		}
	}

	public void insertCoin() {
		switch (state) {
		case SoldOutState:
			System.out.println("you can't insert coin,the machine sold out!");
			break;
		case OnReadyState:
			state = HasCoin;
			System.out
					.println("you have inserted a coin,next,please turn crank!");
			break;
		case HasCoin:
			System.out.println("you can't insert another coin!");

			break;
		case SoldState:
			System.out.println("please wait!we are giving you a candy!");

			break;
		}

	}

	public void returnCoin() {
		switch (state) {
		case SoldOutState:
			System.out
					.println("you can't return,you haven't inserted a coin yet!");
			break;
		case OnReadyState:
			System.out.println("you haven't inserted a coin yet!");
			break;
		case HasCoin:

			System.out.println("coin return!");
			state = OnReadyState;

			break;
		case SoldState:
			System.out.println("sorry,you already have turned the crank!");

			break;
		}

	}

	public void turnCrank() {
		switch (state) {
		case SoldOutState:
			System.out.println("you turned,but there are no candies!");
			break;
		case OnReadyState:
			System.out.println("you turned,but you haven't inserted a coin!");
			break;
		case HasCoin:
			System.out.println("crank turn...!");
			state = SoldState;
			dispense();
			break;
		case SoldState:
			System.out
					.println("we are giving you a candy,turning another get nothing,!");
			break;
		}

	}

	private void dispense() {
		count = count - 1;
		System.out.println("a candy rolling out!");
		if (count > 0) {
			state = OnReadyState;
		} else {
			System.out.println("Oo,out of candies");
			state = SoldOutState;
		}

	}

	public void printstate() {

		switch (state) {
		case SoldOutState:
			System.out.println("***SoldOutState***");
			break;
		case OnReadyState:
			System.out.println("***OnReadyState***");
			break;
		case HasCoin:

			System.out.println("***HasCoin***");

			break;
		case SoldState:
			System.out.println("***SoldState***");
			break;
		}

	}
}

public class MainTest {
	public static void main(String[] args) {
		CandyMachine mCandyMachine=new CandyMachine(1);
		
		mCandyMachine.printstate();
		
		mCandyMachine.insertCoin();
		mCandyMachine.printstate();
		
		mCandyMachine.turnCrank();
		
		mCandyMachine.printstate();
		
		mCandyMachine.insertCoin();
		mCandyMachine.printstate();
		
		mCandyMachine.turnCrank();
		
		mCandyMachine.printstate();
	}
}

加入游戏元素：有10%的概率可以拿到2粒糖果。

以上的设计会违反开闭原则，并且对状态的维护麻烦。

开闭原则：对扩展开放，对修改原有的代码关闭。

状态模式的设计方案：

类图：

状态接口：

public interface State {
	public void insertCoin();
	public void returnCoin();
	public void turnCrank();
	public void dispense();
	public void printstate();
}

状态族类：

public class OnReadyState implements State {
	private CandyMachine mCandyMachine;
	public OnReadyState(CandyMachine mCandyMachine)
	{
		this.mCandyMachine=mCandyMachine;
	}

	@Override
	public void insertCoin() {
		// TODO Auto-generated method stub
		System.out
		.println("you have inserted a coin,next,please turn crank!");
		mCandyMachine.setState(mCandyMachine.mHasCoin);
	}

	@Override
	public void returnCoin() {
		// TODO Auto-generated method stub
		System.out.println("you haven't inserted a coin yet!");
		
	}

	@Override
	public void turnCrank() {
		// TODO Auto-generated method stub
		System.out.println("you turned,but you haven't inserted a coin!");
		
	}

	@Override
	public void dispense() {
		// TODO Auto-generated method stub

	}

	@Override
	public void printstate() {
		// TODO Auto-generated method stub
		System.out.println("***OnReadyState***");
		
	}

}

public class SoldOutState implements State {


	private CandyMachine mCandyMachine;
	public SoldOutState(CandyMachine mCandyMachine)
	{
		this.mCandyMachine=mCandyMachine;
	}


	@Override
	public void insertCoin() {
		// TODO Auto-generated method stub
		System.out.println("you can't insert coin,the machine sold out!");
		
	}


	@Override
	public void returnCoin() {
		// TODO Auto-generated method stub
		System.out
		.println("you can't return,you haven't inserted a coin yet!");


	}


	@Override
	public void turnCrank() {
		// TODO Auto-generated method stub
		System.out.println("you turned,but there are no candies!");
		
	}


	@Override
	public void dispense() {
		// TODO Auto-generated method stub


	}


	@Override
	public void printstate() {
		// TODO Auto-generated method stub
		System.out.println("***SoldOutState***");
	
	}


}

public class SoldState implements State {
	private CandyMachine mCandyMachine;
	public SoldState(CandyMachine mCandyMachine)
	{
		this.mCandyMachine=mCandyMachine;
	}


	@Override
	public void insertCoin() {
		// TODO Auto-generated method stub
		System.out.println("please wait!we are giving you a candy!");


	}


	@Override
	public void returnCoin() {
		// TODO Auto-generated method stub
		System.out.println("you haven't inserted a coin yet!");
		
	}


	@Override
	public void turnCrank() {
		// TODO Auto-generated method stub
		System.out
		.println("we are giving you a candy,turning another get nothing,!");


	}


	@Override
	public void dispense() {
		// TODO Auto-generated method stub
		
		mCandyMachine.releaseCandy();
		if (mCandyMachine.getCount() > 0) {
			mCandyMachine.setState(mCandyMachine.mOnReadyState);
		} else {
			System.out.println("Oo,out of candies");
			mCandyMachine.setState(mCandyMachine.mSoldOutState);
		}


	
	
	}


	@Override
	public void printstate() {
		// TODO Auto-generated method stub
		System.out.println("***SoldState***");
		
	}


}

public class WinnerState implements State {


	private CandyMachine mCandyMachine;


	public WinnerState(CandyMachine mCandyMachine) {
		this.mCandyMachine = mCandyMachine;
	}


	@Override
	public void insertCoin() {
		// TODO Auto-generated method stub
		System.out.println("please wait!we are giving you a candy!");


	}


	@Override
	public void returnCoin() {
		// TODO Auto-generated method stub
		System.out.println("you haven't inserted a coin yet!");


	}


	@Override
	public void turnCrank() {
		// TODO Auto-generated method stub
		System.out
				.println("we are giving you a candy,turning another get nothing,!");


	}


	@Override
	public void dispense() {
		// TODO Auto-generated method stub


		
		mCandyMachine.releaseCandy();
		if (mCandyMachine.getCount() == 0) {
			mCandyMachine.setState(mCandyMachine.mSoldOutState);
		} else {
			System.out.println("you are a winner!you get another candy!");
			mCandyMachine.releaseCandy();
			if (mCandyMachine.getCount() > 0) {
				mCandyMachine.setState(mCandyMachine.mOnReadyState);
			} else {
				System.out.println("Oo,out of candies");
				mCandyMachine.setState(mCandyMachine.mSoldOutState);
			}
		}


	}


	@Override
	public void printstate() {
		// TODO Auto-generated method stub
		System.out.println("***WinnerState***");


	}


}

public class HasCoin implements State {
	private CandyMachine mCandyMachine;


	public HasCoin(CandyMachine mCandyMachine) {
		this.mCandyMachine = mCandyMachine;
	}


	@Override
	public void insertCoin() {
		// TODO Auto-generated method stub
		System.out.println("you can't insert another coin!");


	}


	@Override
	public void returnCoin() {
		// TODO Auto-generated method stub
		System.out.println("coin return!");
		mCandyMachine.setState(mCandyMachine.mOnReadyState);
	}


	@Override
	public void turnCrank() {
		// TODO Auto-generated method stub
		System.out.println("crank turn...!");
		Random ranwinner=new Random();
		int winner=ranwinner.nextInt(10);
		if(winner==0)
		{
			mCandyMachine.setState(mCandyMachine.mWinnerState);


		}else
		{
			mCandyMachine.setState(mCandyMachine.mSoldState);


		}
		
	}


	@Override
	public void dispense() {
	}


	@Override
	public void printstate() {
		// TODO Auto-generated method stub
		System.out.println("***HasCoin***");


	}


}

糖果机类：

public class CandyMachine {

	State mSoldOutState;
	State mOnReadyState;
	State mHasCoin;
	State mSoldState;
	State mWinnerState;
	private State state;
	private int count = 0;

	public CandyMachine(int count) {
		this.count = count;
		mSoldOutState = new SoldOutState(this);
		mOnReadyState = new OnReadyState(this);
		mHasCoin = new HasCoin(this);
		mSoldState = new SoldState(this);
		mWinnerState = new WinnerState(this);
		if (count > 0) {
			state = mOnReadyState;
		} else {
			state = mSoldOutState;
		}
	}

	public void setState(State state) {
		this.state = state;
	}

	public void insertCoin() {
		state.insertCoin();
	}

	public void returnCoin() {
		state.returnCoin();
	}

	public void turnCrank() {
		state.turnCrank();
		state.dispense();
	}

	void releaseCandy() {

		// TODO Auto-generated method stub
		if (count > 0) {
			count = count - 1;
			System.out.println("a candy rolling out!");
		}

	}

	public int getCount() {
		return count;
	}

	public void printstate() {
		state.printstate();
	}
}

public class MainTest {
	public static void main(String[] args) {
		CandyMachine mCandyMachine = new CandyMachine(6);


		mCandyMachine.printstate();


		mCandyMachine.insertCoin();
		mCandyMachine.printstate();


		mCandyMachine.turnCrank();


		mCandyMachine.printstate();


		mCandyMachine.insertCoin();
		mCandyMachine.printstate();


		mCandyMachine.turnCrank();


		mCandyMachine.printstate();
	}
}

状态模式：能根据内部状态的变化，改变对象的行为，看起来好像修改了类。不同的状态对应不同的类文件，增加了系统文件个数。

使用场景

1. 一个对象的行为取决于他的状态，并且它必须在运行时根据状态改变它的行为；
2.一个操作中含有庞大的多分支结构，并且这些分支决定于对象的状态

优缺点

主要优点有：

1.状态模式将不同状态所对应的行为彼此分隔开来，降低程序的耦合，从而在新增或修改状态时，可以避免程序互相影响。

2.状态模式将状态的逻辑处理变化交由上下文对象Context管理，便于客户端的调用

 

缺点主要有：

不同的状态对应不同的类文件，增加了系统文件个数，不便于维护管理。