我原本以为在对乡村的一切事物都很容易,但是每次我一回头就有更多变更的请求纷至沓来。我快崩溃了!----当小编读到这段话的时候,甚是感慨,设计模式的世界真的不是那么容易弄懂的,好在我还在学习。
基本常识:策略模式和状态模式是双胞胎,在出生时才分开。策略模式是围绕可以互换的算法来创建成功业务的。然而,状态走的是更崇高的路,它通过改变对象内部的状态来帮助对象控制自己的行为。
我们先来看下对象村的大家,这次又想出了什么。他们发现只要在糖果机里装上CPU,就可以增加销量、通过网络监测库存,并且能准确地得知客户的满意度。所有,就有了下面的这个设想:
状态机101
我们如何从状态图得到真正的代码呢?下面是一个实现状态机的简单介绍
- 首先,找出所有的状态:
我们有:没有25分钱、有25分钱、糖果售罄、售出糖果四个状态
- 接下来,创建一个实例变量来持有目前的状态,然后定义每个状态的值:
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;
- 现在,我们将所有系统中可以发生的动作整合起来:
投入25分钱、退回25分钱、转动曲柄、发放糖果
- 现在,我们创建一个类,它的作用就像是一个状态机。对每一个动作,我们都创建了一个对应的方法,这些方法利用条件语句来决定在每个状态内什么行为是恰当的。比如对“投入25分钱”这个动作来说,我们可以把对应方法写成下面的样子:
public void insertQuarter() {
if (state == HAS_QUARTER) {
System.out.println("You can't insert another quarter");
} else if (state == NO_QUARTER) {
state = HAS_QUARTER;
System.out.println("You inserted a quarter");
} else if (state == SOLD_OUT) {
System.out.println("You can't insert a quarter, the machine is old out");
} else if (state == SOLD) {
System.out.println("Please wait, we're already giving you a gumball");
}
}
写下代码
现在我们来实现糖果机。我们知道要利用实例变量持有当前的状态,然后需要处理所有可能发生的动作、行为和状态的转换。我们需要实现的动过包括:投入25分钱、退回25分钱、转动曲柄和发放糖果;也要检查糖果是否售罄。
public class GumballMachine {
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;
int count = 0;
public GumballMachine(int count) {
this.count = count;
if (count > 0) {
state = NO_QUARTER;
}
}
// 当有25分钱投进来,就会执行这里
public void insertQuarter() {
if (state == HAS_QUARTER) {
System.out.println("You can't insert another quarter");
} else if (state == NO_QUARTER) {
state = HAS_QUARTER;
System.out.println("You inserted a quarter");
} else if (state == SOLD_OUT) {
System.out.println("You can't insert a quarter, the machine is sold out");
} else if (state == SOLD) {
System.out.println("Please wait, we're already giving you a gumball");
}
}
// 现在,如果顾客试着退回25分钱
public void ejectQuarter() {
if (state == HAS_QUARTER) {
System.out.println("Quarter returned");
state = NO_QUARTER;
} else if (state == NO_QUARTER) {
System.out.println("You haven't inserted a quarter");
} else if (state == SOLD) {
System.out.println("Sorry, you already turned the crank");
} else if (state == SOLD_OUT) {
System.out.println("You can't eject, you haven't inserted a quarter yet");
}
}
// 顾客试着转动曲柄
public void turnCrank() {
if (state == SOLD) {
System.out.println("Turning twice doesn't get you another gumball!");
} else if (state == NO_QUARTER) {
System.out.println("You turned but there's no quarter");
} else if (state == SOLD_OUT) {
System.out.println("You turned, but there are no gumballs");
} else if (state == HAS_QUARTER) {
System.out.println("You turned...");
state = SOLD;
dispense();
}
}
// 调用此方法,发放糖果
private void dispense() {
if (state == SOLD) {
System.out.println("A gumball comes rolling out the slot");
count = count - 1;
if (count == 0) {
System.out.println("Oops, out of gumballs!");
state = SOLD_OUT;
} else {
state = NO_QUARTER;
}
} else if (state == NO_QUARTER) {
System.out.println("You need to pay first");
} else if (state == SOLD_OUT) {
System.out.println("No gumball dispensed");
} else if (state == HAS_QUARTER) {
System.out.println("No gumball dispensed");
}
}
public void refill(int numGumBalls) {
this.count = numGumBalls;
state = NO_QUARTER;
}
public String toString() {
StringBuffer result = new StringBuffer();
result.append("
Mighty Gumball, Inc.");
result.append("
Java-enabled Standing Gumball Model #2004
");
result.append("Inventory: " + count + " gumball");
if (count != 1) {
result.append("s");
}
result.append("
Machine is ");
if (state == SOLD_OUT) {
result.append("sold out");
} else if (state == NO_QUARTER) {
result.append("waiting for quarter");
} else if (state == HAS_QUARTER) {
result.append("waiting for turn of crank");
} else if (state == SOLD) {
result.append("delivering a gumball");
}
result.append("
");
return result.toString();
}
}
现在,让我们进入一个小小的内部测试,看下是否符合了要求。
public class WinnerState implements State {
GumballMachine gumballMachine;
public WinnerState(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("Please wait, we're already giving you a Gumball");
}
public void turnCrank() {
System.out.println("Turning again doesn't get you another gumball!");
}
public void dispense() {
gumballMachine.releaseBall();
if (gumballMachine.getCount() == 0) {
gumballMachine.setState(gumballMachine.getSoldOutState());
} else {
gumballMachine.releaseBall();
System.out.println("YOU'RE A WINNER! You got two gumballs for your quarter");
if (gumballMachine.getCount() > 0) {
gumballMachine.setState(gumballMachine.getNoQuarterState());
} else {
System.out.println("Oops, out of gumballs!");
gumballMachine.setState(gumballMachine.getSoldOutState());
}
}
}
public void refill() { }
public String toString() {
return "despensing two gumballs for your quarter, because YOU'RE A WINNER!";
}
}
运行结果比较长,在这里就不进行打印啦,需要的朋友请自行根据代码进行执行。
变更需求来了,需要新设计
现在,客户要求当曲柄转动时,有10%的几率掉下来的是两颗糖果(多送你一颗),那你应该怎么做呢?
我们的计划是这样的:不要维护我们现有的代码,我们重写它以便于将状态对象封装在各自的类中,然后在动作发生时委托给当前状态。所以,我们要做的事情就是:
- 首先,我们定义一个State接口。在这个接口中,糖果机的每个动作都有一个对应的方法
- 然后为机器中的每个状态实现状态类。这些将负责在对应的状态下进行机器的行为
- 最后,我们要摆脱旧的条件代码,取而代之的方式是,将动作委托到状态类
你将会看到,我们不仅遵守了设计原则,实际上我们还实现了状态模式。接下来,我们定义状态接口和类,如下:
好了,到现在为止,我们下一步肯定是要去实现怎么改造了,对吧。在这里呢,给大家先留着,先去思考思考,如何进行改造。思路有了,类图有了,就看你怎么想啦。我们下一次揭晓。