【設計模式】之狀態模式（State）

狀態模式的定義是：當一個對象內部狀態發生變化時，允許它改變自己的行為。這個對象好像可以改變它的類。

The English Definition：Allow an object to change its behavior when its internal state changes. The object will appear to change its class.

簡單來說就是，當一個對象的內部狀態發生變化時，它會自動的改變自己的行為，這些變化對用戶端的調用代碼是透明的。

其實就是一個狀態機器的概念，運用狀態模型，可以避免使用過多的條件判斷語句，而且當狀態增加以後，可以很容易地添加修改代碼。

狀態模式的應用比較廣泛，網路通訊協定、繪圖工具等。

以下代碼代碼來自《Head First Design Patterns》：

首先定義一個State的抽象類別

public interface State {    public void insertQuarter();    public void ejectQuarter();    public void turnCrank();    public void dispense();}

然後是具體的狀態類

public class SoldOutState implements State {    GumballMachine gumballMachine;    public SoldOutState(GumballMachine gumballMachine) {        this.gumballMachine = gumballMachine;    }    public void insertQuarter() {        System.out.println("You can't insert a quarter, the machine is sold out");    }    public void ejectQuarter() {        System.out.println("You can't eject, you haven't inserted a quarter yet");    }    public void turnCrank() {        System.out.println("You turned, but there are no gumballs");    }    public void dispense() {        System.out.println("No gumball dispensed");    }}

import java.util.Random;public class HasQuarterState implements State {    Random randomWinner = new Random(System.currentTimeMillis());    GumballMachine gumballMachine;    public HasQuarterState(GumballMachine gumballMachine) {        this.gumballMachine = gumballMachine;    }    public void insertQuarter() {        System.out.println("You can't insert another quarter");    }    public void ejectQuarter() {        System.out.println("Quarter returned");        gumballMachine.setState(gumballMachine.getNoQuarterState());    }    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());        }    }    public void dispense() {        System.out.println("No gumball dispensed");    }}

public class SoldState implements State {    GumballMachine gumballMachine;    public SoldState(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("Sorry, you already turned the crank");    }    public void turnCrank() {        System.out.println("Turning twice doesn't get you another gumball!");    }    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());        }    }}

public class NoQuarterState implements State {    GumballMachine gumballMachine;    public NoQuarterState(GumballMachine gumballMachine) {        this.gumballMachine = gumballMachine;    }    public void insertQuarter() {        System.out.println("You inserted a quarter");        gumballMachine.setState(gumballMachine.getHasQuarterState());    }    public void ejectQuarter() {        System.out.println("You haven't inserted a quarter");    }    public void turnCrank() {        System.out.println("You turned, but there's no quarter");    }    public void dispense() {        System.out.println("You need to pay first");    }}

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("Sorry, you already turned the crank");    }    public void turnCrank() {        System.out.println("Turning twice doesn't get you another gumball!");    }    public void dispense() {        System.out.println("YOU'RE A WINNER! You get two gumballs for your quarter");        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());            }        }    }}

然後定義主體類（Context），它的行為就是通過狀態見的轉換來控制的。

public class GumballMachine {    State soldOutState;    State noQuarterState;    State hasQuarterState;    State soldState;    State winnerState;    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);        winnerState = new WinnerState(this);        if (numberGumballs > 0) {            state = noQuarterState;        }        count = numberGumballs;    }    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 = count - 1;        }    }    State getSoldOutState() {        return soldOutState;    }    State getNoQuarterState() {        return noQuarterState;    }    State getHasQuarterState() {        return hasQuarterState;    }    State getSoldState() {        return soldState;    }    State getWinnerState() {        return winnerState;    }    int getCount() {        return count;    }}

測試代碼

public class GumballMachineTestDrive {    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.turnCrank();        gumballMachine.insertQuarter();        gumballMachine.turnCrank();        System.out.println(gumballMachine);    }}