设计模式之状态模式（一）

时间 2019-12-10

标签设计模式状态繁體版

原文原文链接

我本来觉得在对乡村的一切事物都很容易，可是每次我一回头就有更多变动的请求纷至沓来。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。java

基本常识：策略模式和状态模式是双胞胎，在出生时才分开。策略模式是围绕能够互换的算法来建立成功业务的。然而，状态走的是更崇高的路，它经过改变对象内部的状态来帮助对象控制本身的行为。算法

咱们先来看下对象村的你们，此次又想出了什么。他们发现只要在糖果机里装上CPU，就能够增长销量、经过网络监测库存，而且能准确地得知客户的满意度。全部，就有了下面的这个设想：设计模式

状态机101

咱们如何从状态图获得真正的代码呢？下面是一个实现状态机的简单介绍网络

首先，找出全部的状态：

咱们有：没有25分钱、有25分钱、糖果售罄、售出糖果四个状态app

接下来，建立一个实例变量来持有目前的状态，而后定义每一个状态的值:

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;

如今，咱们将全部系统中能够发生的动做整合起来：

投入25分钱、退回25分钱、转动曲柄、发放糖果学习

如今，咱们建立一个类，它的做用就像是一个状态机。对每个动做，咱们都建立了一个对应的方法，这些方法利用条件语句来决定在每一个状态内什么行为是恰当的。好比对“投入25分钱”这个动做来讲，咱们能够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

如今咱们来实现糖果机。咱们知道要利用实例变量持有当前的状态，而后须要处理全部可能发生的动做、行为和状态的转换。咱们须要实现的动过包括：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。测试

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");
        }
    }

    // 如今，若是顾客试着退回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("\nMighty Gumball, Inc.");
        result.append("\nJava-enabled Standing Gumball Model #2004\n");
        result.append("Inventory: " + count + " gumball");
        if (count != 1) {
            result.append("s");
        }
        result.append("\nMachine 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("\n");
        return result.toString();
    }
}

如今，让咱们进入一个小小的内部测试，看下是否符合了要求。this

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，须要的朋友请自行根据代码进行执行。设计

变动需求来了，须要新设计

如今，客户要求当曲柄转动时，有10%的概率掉下来的是两颗糖果（多送你一颗），那你应该怎么作呢？code

咱们的计划是这样的：不要维护咱们现有的代码，咱们重写它以便于将状态对象封装在各自的类中，而后在动做发生时委托给当前状态。因此，咱们要作的事情就是：

首先，咱们定义一个State接口。在这个接口中，糖果机的每一个动做都有一个对应的方法
而后为机器中的每一个状态实现状态类。这些将负责在对应的状态下进行机器的行为
最后，咱们要摆脱旧的条件代码，取而代之的方式是，将动做委托到状态类

你将会看到，咱们不只遵照了设计原则，实际上咱们还实现了状态模式。接下来，咱们定义状态接口和类，以下：

好了，到如今为止，咱们下一步确定是要去实现怎么改造了，对吧。在这里呢，给你们先留着，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。咱们下一次揭晓。

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