本文出自
基本常识: 策略模式和状态模式是双胞胎,在出生时才分开
认识状态模式
假设有一个糖果机, 它的工作状态图如下:
要用代码实现糖果机的功能, 如果不用状态模式:
一种方法是创建一个类,它的作用就是一个状态机,对每一个动作,我们都创建了一个对应的方法,这些方法用条件语句来决定在每一个状态内什么方法是最恰当的.比如对"投入25分钱"这个动作,对应的方法如下:
// 我们根据糖果机的状态,定义4种状态,用整形常量来表示final static int SOLD_OUT= 0; // 糖果卖完了final static int NO_QUARTER = 1; // 没有投钱final static int HAS_QUARTER = 2; // 已投钱了final static int SOLD = 3; // 正在出售糖果public void insertQurter() { // 根据不同的状态,会有不同的动作反应 if (state == HAS_QUARTER) { // do something } else if (state == SOLD_OUT) { // do something } else if (state == SOLD) { // do something } else if (state == NO_QUARTER) { // do something }}
这样做的缺点: 会产生大量的if...else语句, 代码将很不容易改变, 难以拓展.
没有遵守"开放-关闭"原则
不符合面向对象
状态转换隐藏在条件语句中,所以并不明显
没有把会改变的部分包装起来
未来加入的代码可能导致bug
是时候学习新的设计模式了
定义状态模式
状态模式允许对象在内部状态改变时改变它的行为,对象看起来好像修改了它的类
因为这个模式将状态封装成为独立的类，并将动作委托到代表当前状态的对象，我们知道行为会随着内部状态而改变。
Contex（上下文）：是一个类，它可以拥有一些内部状态。
State接口：定义了一个所有具体状态的共同接口；任何状态都实现这个相同的接口，这样一来，状态之间可以互相替换
ConcreteState(具体状态): 处理来自Context的请求.每个ConcreteState都提供了它自己对于请求的实现.所以,当Context改变状态时行为也跟着改变
状态模式的类图和策略模式的基本一模一样, 但它们的"意图"不一样
状态模式允许Context随着状态的改变而改变行为.
策略模式通常会用行为或算法来配置Context类. 通常某个Context类只有一个最适当的策略.
使用状态模式通常那个会导致设计中的类数目大量增加. 因为"一个类,一个责任"原则
用状态模式实现糖果机
状态类图:
1. 实现State接口
public interface State { // 封装四种动作 public void insertQuarter(); // 投币 public void ejectQuarter(); // 退币 public void turnCrank(); // 转动摇柄 public void dispense(); // 发糖果} 2. 实现具体的状态 // 没有投币的状态public class NoQuarterState implements State { GumballMachine gumballMachine; public NoQuarterState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } public void insertQuarter() { System.out.println("You inserted a quarter"); gumballMachine.setState(gumballMachine.getHasQuarterState()); } public void ejectQuarter() { System.out.println("You haven't inserted a quarter"); } public void turnCrank() { System.out.println("You turned, but there's no quarter"); } public void dispense() { System.out.println("You need to pay first"); } public String toString() { return "waiting for quarter"; }} // 已经投币的状态public class HasQuarterState implements State { GumballMachine gumballMachine; public HasQuarterState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } public void insertQuarter() { System.out.println("You can't insert another quarter"); } public void ejectQuarter() { System.out.println("Quarter returned"); gumballMachine.setState(gumballMachine.getNoQuarterState()); } public void turnCrank() { System.out.println("You turned..."); gumballMachine.setState(gumballMachine.getSoldState()); } public void dispense() { System.out.println("No gumball dispensed"); } public String toString() { return "waiting for turn of crank"; }} // 正在售出的状态public class SoldState implements State { GumballMachine gumballMachine; public SoldState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } public void insertQuarter() { System.out.println("Please wait, we're already giving you a gumball"); } public void ejectQuarter() { System.out.println("Sorry, you already turned the crank"); } public void turnCrank() { System.out.println("Turning twice doesn't get you another gumball!"); } public void dispense() { gumballMachine.releaseBall(); if (gumballMachine.getCount() > 0) { gumballMachine.setState(gumballMachine.getNoQuarterState()); } else { System.out.println("Oops, out of gumballs!"); gumballMachine.setState(gumballMachine.getSoldOutState()); } } public String toString() { return "dispensing a gumball"; }} ..... 3. 实现Context
public class GumballMachine { State soldOutState; State noQuarterState; State hasQuarterState; State soldState; State state = soldOutState; int count = 0; public GumballMachine(int numberGumballs) { soldOutState = new SoldOutState(this); noQuarterState = new NoQuarterState(this); hasQuarterState = new HasQuarterState(this); soldState = new SoldState(this); this.count = numberGumballs; if (numberGumballs > 0) { state = noQuarterState; } } public void insertQuarter() { state.insertQuarter(); } public void ejectQuarter() { state.ejectQuarter(); } public void turnCrank() { state.turnCrank(); state.dispense(); } void setState(State state) { this.state = state; } void releaseBall() { System.out.println("A gumball comes rolling out the slot..."); if (count != 0) { count = count - 1; } } int getCount() { return count; } void refill(int count) { this.count = count; state = noQuarterState; } public State getState() { return state; } public State getSoldOutState() { return soldOutState; } public State getNoQuarterState() { return noQuarterState; } public State getHasQuarterState() { return hasQuarterState; } public State getSoldState() { return soldState; } public String toString() { StringBuffer result = new StringBuffer(); result.append("\nMighty Gumball, Inc."); result.append("\nJava-enabled Standing Gumball Model #2004"); result.append("\nInventory: " + count + " gumball"); if (count != 1) { result.append("s"); } result.append("\n"); result.append("Machine is " + state + "\n"); return result.toString(); }} 4. 测试类
public class GumballMachineTestDrive { public static void main(String[] args) { GumballMachine gumballMachine = new GumballMachine(5); System.out.println(gumballMachine); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); System.out.println(gumballMachine); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); System.out.println(gumballMachine); }} 这样做的优点: 1. 将每个状态的行为局部化到自己的类中
2. 将容易产生问题的if语句删除,以方便日后的维护
3. 让每个状态"对修改关闭",让糖果机"对拓展开放",因为可以加入新的状态类
4. 创建一个新的代码基和类结构,这更能映射糖果的图,而且容易阅读和理解