《HeadFirst设计模式(第二版)》第十章代码—

如下图所示，这是一个糖果机的状态机图，要求使用代码实现：

初始版本：

package Chapter10_StatePattern.Origin;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

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;
        }
    }

    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");
        }
    }

    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");
        }
    }

    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();
        }
    }

    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();
    }
}

上面的代码很明显极其不利于本维护：比如要添加一个新的状态--每次投入25分钱都能有十分之一的概率获得两个糖果。

很明显上面的代码又要添加一堆if-else了，这种情况下就可以使用状态模式：

状态模式：

允许对象内部状态改变时改变其行为。对象看起来好像改变了他的类。

使用了状态模式之后的类图：

State

package Chapter10_StatePattern.State;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

public interface State {

    public void insertQuarter();//投入25分
    public void ejectQuarter();//吐出25分钱
    public void turnCrank();//摇动摇杆
    public void dispense();//发放
    public void refill();//填充糖果
}

接下来是五个状态类

NoQuarterState

package Chapter10_StatePattern.State;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

//没有25美分
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 void refill() { }

    public String toString() {
        return "waiting for quarter";
    }
}

HasQuarterState

package Chapter10_StatePattern.State;

import java.util.Random;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

//有25美分
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 void refill() { }

    public String toString() {
        return "waiting for turn of crank";
    }
}

SoldState

package Chapter10_StatePattern.State;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/
//糖果已售出
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 void refill() { }

    public String toString() {

        return "dispensing a gumball";
    }
}

SoldOutState

package Chapter10_StatePattern.State;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

//糖果已售罄
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");
    }

    public void refill() {
        gumballMachine.setState(gumballMachine.getNoQuarterState());
    }

    public String toString() {
        return "sold out";
    }
}

WinnerState

package Chapter10_StatePattern.State;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

//获得两个糖果
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 "dispensing two gumballs for your quarter, because YOU'RE A WINNER!";
    }

}

GumballMachine:

package Chapter10_StatePattern.State;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

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);

        this.count = numberGumballs;
        if (numberGumballs > 0) {
            state = noQuarterState;
        }
    }

    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;
        }
    }

    int getCount() {
        return count;
    }

    void refill(int count) {
        this.count += count;
        System.out.println("The gumball machine was just refilled; its new count is: " + this.count);
        state.refill();
    }

    public State getState() {
        return state;
    }

    public State getSoldOutState() {
        return soldOutState;
    }

    public State getNoQuarterState() {
        return noQuarterState;
    }

    public State getHasQuarterState() {
        return hasQuarterState;
    }

    public State getSoldState() {
        return soldState;
    }

    public State getWinnerState() {
        return winnerState;
    }

    public String toString() {
        StringBuffer result = new StringBuffer();
        result.append("\nMighty Gumball, Inc.");
        result.append("\nJava-enabled Standing Gumball Model #2004");
        result.append("\nInventory: " + count + " gumball");
        if (count != 1) {
            result.append("s");
        }
        result.append("\n");
        result.append("Machine is " + state + "\n");
        return result.toString();
    }
}

GumballMachineTestDrive

package Chapter10_StatePattern.State;

import Chapter10_StatePattern.Origin.GumballMachine;

/**
 * @Author 竹心
 * @Date 2023/8/19
 **/

public class GumballMachineTestDrive {
    public static void main(String[] args) {
        GumballMachine gumballMachine =
                new GumballMachine(10);

        System.out.println(gumballMachine);

        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();

        System.out.println(gumballMachine);

        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();

        System.out.println(gumballMachine);

        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();

        System.out.println(gumballMachine);

        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();

        System.out.println(gumballMachine);

        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();

        System.out.println(gumballMachine);
    }
}