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今天我们来探讨一下设计模式中的状态模式。
假设我们需要完成这样一个需求，设计一个糖果机程序：
顾客需要往糖果机投入硬币，然后摇动糖果机把手，糖果机再给顾客吐糖果。
这里顾客有以下动作：
1）投硬币
2）取消，吐回硬币
2）转动把手
3）拿到糖果
糖果机也有几种状态：
1）待机
2）收到硬币
3）吐完糖果
4）售罄
顾客所做的动作，根据糖果机状态的不同而有不同的结果。
状态图如下：
传统的做法是，顾客的每个行为对应一个函数，在函数中使用switch根据不同的状态执行不同的代码。如下。
public class CandyMachine {
final static int SoldOutState = 0;//卖完
final static int OnReadyState = 1;//待机
final static int HasCoin = 2;//有硬币
final static int SoldState = 3;//吐完糖果
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;
}
}
}
这样完全满足需求中的描述。
新需求：加入游戏元素，每次摇动把手都有10%的概率拿到2颗糖果。
点评：传统的做法显然是不符合开闭原则的，如果有新需求将需要修改已有的代码，为了满足新需求，需要在每个动作中加入新的switch分支，因此破坏了对修改关闭的原则。这不是好的设计。好的设计应该是针对接口的开发，而不是功能的开发。在做设计的时候需要思考那些是改变的，那些是不变的。
通过分析，我们发现动作是不变的，而状态可能会增加。因此，我们可以通过在接口中定义不变的动作，新的状态只需要增加相应的状态实现类即可。
下面，介绍状态模式如何解决以上的问题。
状态模式的定义：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。
通过状态模式重构的类图如下。
原来的状态由整数变成对象。代码如下。
public interface State {
public void insertCoin();
public void returnCoin();
public void turnCrank();
public void dispense();
public void printstate();
}
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 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 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***");
}
}
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();
}
}
测试类
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();
}
}
点评：根据顾客动作，在不同状态间切换。需要有新的状态时候，只需要增加类即可，因为动作是不变的。
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原文地址：http://my.oschina.net/gaohongtian/blog/491843
