设计模式之状态模式
情景：有一个自动投币的糖果机。它有以下几种状态（每个圆圈代表一个状态）：
如果用常量值来定义几个变量
final static int SOLD_OUT = 0; final static int NO_QUARTER = 1; final static int HAS_QUARTER = 2; final static int SOLD = 3; int state = SOLD_OUT;
这样状态变化就可以写成：
public void insertQuarter() { if (state == HAS_QUARTER) { ... } else if (state == ...) { ... } ... }
每一个动作都要先判断状态，改后再决定反馈以及下一个状态是什么。
虽然可以实现要求，但是当出现新的状态的时候，很不利于扩展。
我们把状态变成一个类，把不同的状态封装在各自的类中，然后在动作发生时委托给当前状态。
首先，定义状态接口
public interface State { public void insertQuarter(); public void ejectQuarter(); public void turnCrank(); public void dispense(); }
然后定义各种状态
public class NoQuarterState implements State { GumballMachine gumballMachine; public NoQuarterState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } @Override public void insertQuarter() { System.out.println("You insert a quarter"); gumballMachine.setState(gumballMachine.getHasQuarterState()); } @Override public void ejectQuarter() { System.out.println("You haven't inserted a quarter"); } @Override public void turnCrank() { System.out.println("You turned, but there's no quarter"); } @Override public void dispense() { System.out.println("You need to pay first"); } }
public class HasQuarterState implements State { GumballMachine gumballMachine; public HasQuarterState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } @Override public void insertQuarter() { System.out.println("You can't insert another quater"); } @Override public void ejectQuarter() { System.out.println("Quarter returned"); gumballMachine.setState(gumballMachine.getNoQuarterState()); } @Override public void turnCrank() { System.out.println("You turned..."); gumballMachine.setState(gumballMachine.getSoldState()); } @Override public void dispense() { System.out.println("No gumball dispensed"); } }
public class SoldOutState implements State { GumballMachine gumballMachine; public SoldOutState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } @Override public void insertQuarter() { System.out.println("You can't insert a quarter, the machine is sold out"); } @Override public void ejectQuarter() { System.out.println("You can't eject, you haven't inserted a quarter yet"); } @Override public void turnCrank() { System.out.println("You turned, but there are no gumballs"); } @Override public void dispense() { System.out.println("No gumball dispensed"); } }
public class SoldState implements State { GumballMachine gumballMachine; public SoldState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } @Override public void insertQuarter() { System.out.println("Please wait, we're already giving you a gumball"); } @Override public void ejectQuarter() { System.out.println("Sorry, you already turned the crank"); } @Override public void turnCrank() { System.out.println("Turning twice doesn't get you another gumball!"); } @Override 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()); } } }
最后写一下糖果机
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); 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 -= 1; } } public int getCount() { return count; } public State getSoldOutState() { return soldOutState; } public State getNoQuarterState() { return noQuarterState; } public State getHasQuarterState() { return hasQuarterState; } public State getSoldState() { return soldState; } public String toString() { return "\nMighty Gumball, Inc\n" + "Inventory: " + count + " gumballs\n"; } }
测试类：
public class GumballMachineTestDriver { 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.ejectQuarter(); gumballMachine.turnCrank(); System.out.println(gumballMachine); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); gumballMachine.ejectQuarter(); System.out.println(gumballMachine); gumballMachine.insertQuarter(); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); gumballMachine.insertQuarter(); gumballMachine.turnCrank(); System.out.println(gumballMachine); } }
将每一个状态局部化到它自己的类中。将容易产生问题的if语句删除，方便日后的维护。
这样，每一个状态“对修改关闭”，糖果机“对扩展开放”
状态模式：允许对象在内部状态改变时改变他的行为，对象看起来好像修改了它的类。
类图：
现在我们改变需求，希望有10%的概率中奖，也就是投一个币，获得两个糖果。
我们只需要添加一个新的状态类。
public class WinnerState implements State { GumballMachine gumballMachine; public WinnerState(GumballMachine gumballMachine) { this.gumballMachine = gumballMachine; } @Override public void insertQuarter() { System.out.println("Please wait, we're already giving you a gumball"); } @Override public void ejectQuarter() { System.out.println("Sorry, you already turned the crank"); } @Override public void turnCrank() { System.out.println("Turning twice doesn't get you another gumball!"); } @Override public void dispense() { System.out.println("YOU'RE A WINNER! YOU GET TWO GUMBALLS FOR YOUR QUARTERS"); gumballMachine.releaseBall(); if (gumballMachine.getCount() == 0) { gumballMachine.setState(gumballMachine.getSoldOutState()); } else { gumballMachine.releaseBall(); if (gumballMachine.getCount() > 0) { gumballMachine.setState(gumballMachine.getNoQuarterState()); } else { System.out.println("Oops, out of gumballs!"); gumballMachine.setState(gumballMachine.getSoldOutState()); } } } }
在HasQuarterState中有几率跳到WinnerState状态：
public class HasQuarterState implements State { GumballMachine gumballMachine; Random randomWinner = new Random(System.currentTimeMillis()); // .... @Override public void turnCrank() { System.out.println("You turned..."); int winner = randomWinner.nextInt(10); if (winner == 0 && (gumballMachine.getCount() > 1)) { gumballMachine.setState(gumballMachine.getWinnerState()); } else { gumballMachine.setState(gumballMachine.getSoldState()); } } // .... }
记得在糖果机类中添加一个WinnerState状态类，这个新的需求就实现了。
- 状态模式允许Context随着状态的改变而改变行为。
- 状态装换可以由State类或者Context类控制。
- 使用状态模式通常会导致设计中的类数目大量增加。
- 状态类可以被多个Context实例共享。
