设计模式系列之状态模式【十一】
1. 前言
《Head First设计模式》中关于状态模式的例子举得很好，如下：
假设我们现在有一个糖果机项目，那么我们知道一般糖果机提供给用户的行为有这么几种：投入硬币、转动曲柄、退出硬币几种行为；糖果机一般有这几种状态：待机状态、持有硬币的准备状态、运行状态即正在售出状态和初始状态。 我们发现处于不同状态的时候，持有的行为是不一样的：
如果采用传统的方法来写代码，那么在投入硬币这个行为操作的时候，会进行状态的判断，只有处于待机状态下是正常的，而其他则非正常，那么其他行为也一样，都需要去先判断下当前的状态来进行操作。得到的代码则为：
package com.linyf.demo.pattern.state;
public class CandyMachine {
final static int SoldOutState = 0; //初始状态
final static int OnReadyState = 1; //待机状态
final static int HasCoin = 2; //准备状态
final static int SoldState = 3; //售出状态
private int state = SoldOutState; //变量,用于存放当前的状态值
private int count = 0; //糖果的数目
public CandyMachine(int count) {
this.count = count;
if (count > 0) {
state = OnReadyState;
}
}
//投入硬币行为的时候，通过判断当前的状态来匹配所有的状态.
public void insertCoin() {
switch (state) {
case SoldOutState:
System.out.println("you can't insert coin,the machine sold out!");
break;
case OnReadyState: //只有在待机状态的时候,投入硬币行为正确,并将状态改变为准备状态
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;
}
}
//回退硬币
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;
}
}
//转动曲柄
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;
}
}
//触发发放糖果行为
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;
}
}
}
上面这种方式存在的问题：
- 存在大量的switch case 语句
- 可扩展性差，并且一旦要加入一种新的状态，那么就会要修改所有的switch case，不符合开闭原则
- 没有采用面向对象的方式去封装
比如，这个时候，新增加了一种状态，赢家状态，即可以获取到两粒糖果；那么如果用上面的方式，肯定是不符合开闭原则的，同时扩展性也不好。
2. 定义
为了解决上面的问题，首先分析项目中变化的部分和不变的部分，抽出变化的部分，我们发现糖果机提供的行为一般是不变的，就是投入硬币、转动曲柄、退回硬币、发放糖果；而糖果机的状态是可以变化的，可以新增出一种状态来，比如赢家状态。于是出现了下面的结构设计方案：
从结构图可以看出，提炼出状态接口，然后将各个状态抽出，并去实现接口，每个状态都持有投入硬币，退回硬币，转动曲柄、售出糖果这几种行为对应的方法做出相应；而糖果机持有所有的状态，并通过引用状态接口来操作各个状态；这种设计架构就是状态模式。
状态模式定义：对象行为的变化是由于状态的变化，那么当内部状态发生变化的时候，就会改变对象的行为，而这种改变似乎就改变了整个类。
3. 重构
现在采用状态模式来解决问题：
3.1 定义接口
public interface State {
public void insertCoin();
public void returnCoin();
public void turnCrank();
public void dispense();
public void printstate();
}
3.2 定义各个状态的实现类
3.2.1 待机状态
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***");
}
}
说明：我们会发现里面存在一个糖果机的属性，而之所以存在这个属性，就是因为糖果机中持有所有的状态，而在准备状态下，肯定会由于某种行为发生状态改变，而要改变的状态都在糖果机中，所以持有一个糖果机属性，下面也一样，不在重复说明。
3.2.2 准备状态
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***");
}
}
3.2.3 初始状态
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***");
}
}
3.2.4 售出状态
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***");
}
}
3.2.5 赢家状态
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***");
}
}
3.3 糖果机
糖果机要持有所有的状态，并在初始化的时候，要设置其开始的状态，然后糖果的各个行为，就委托到了各个状态中自己维护，代码如下：
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();
}
}
3.4 测试
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();
}
}