• 设计模式之状态模式(一)


    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

    1. 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

    1. 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;
    
    1. 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

    1. 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:

    1. 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
    2. 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
    3. 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。

    爱生活,爱学习,爱感悟,爱挨踢

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  • 原文地址:https://www.cnblogs.com/dimple91/p/11231810.html
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