設計模式之狀態模式（一）_ZenDei技術網路在線

我原本以為在對鄉村的一切事物都很容易，但是每次我一回頭就有更多變更的請求紛至沓來。我快崩潰了！當小編讀到這段話的時候，甚是感慨，設計模式的世界真的不是那麼容易弄懂的，好在我還在學習。基本常識：策略模式和狀態模式是雙胞胎，在出生時才分開。策略模式是圍繞可以互換的演算法來創建成功業務的。然而，狀態走 ...

我原本以為在對鄉村的一切事物都很容易，但是每次我一回頭就有更多變更的請求紛至沓來。我快崩潰了！----當小編讀到這段話的時候，甚是感慨，設計模式的世界真的不是那麼容易弄懂的，好在我還在學習。

基本常識：策略模式和狀態模式是雙胞胎，在出生時才分開。策略模式是圍繞可以互換的演算法來創建成功業務的。然而，狀態走的是更崇高的路，它通過改變對象內部的狀態來幫助對象控制自己的行為。

我們先來看下對象村的大家，這次又想出了什麼。他們發現只要在糖果機里裝上CPU，就可以增加銷量、通過網路監測庫存，並且能準確地得知客戶的滿意度。所有，就有了下麵的這個設想：

狀態機101

我們如何從狀態圖得到真正的代碼呢？下麵是一個實現狀態機的簡單介紹

首先，找出所有的狀態：

我們有：沒有25分錢、有25分錢、糖果售罄、售出糖果四個狀態

接下來，創建一個實例變數來持有目前的狀態，然後定義每個狀態的值:

    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;

現在，我們將所有系統中可以發生的動作整合起來：

投入25分錢、退回25分錢、轉動曲柄、發放糖果

現在，我們創建一個類，它的作用就像是一個狀態機。對每一個動作，我們都創建了一個對應的方法，這些方法利用條件語句來決定在每個狀態內什麼行為是恰當的。比如對“投入25分錢”這個動作來說，我們可以把對應方法寫成下麵的樣子:

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");
    }
}

寫下代碼

現在我們來實現糖果機。我們知道要利用實例變數持有當前的狀態，然後需要處理所有可能發生的動作、行為和狀態的轉換。我們需要實現的動過包括：投入25分錢、退回25分錢、轉動曲柄和發放糖果；也要檢查糖果是否售罄。

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;
        }
    }
    // 當有25分錢投進來，就會執行這裡
    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");
        }
    }

    // 現在，如果顧客試著退回25分錢
    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");
        }
    }
 
    // 顧客試著轉動曲柄
    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();
        }
    }
 
    // 調用此方法，發放糖果
    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();
    }
}

現在，讓我們進入一個小小的內部測試，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!";
    }
}

運行結果比較長，在這裡就不進行列印啦，需要的朋友請自行根據代碼進行執行。