设计模式——状态模式（State Pattern）

状态模式允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它的类。

类图

（图片源于网络）

代码实现（Java）

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

// SoldState.java
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 String toString() {
        return "dispensing a gumball";
    }
}

// SoldOutState.java
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 String toString() {
        return "sold out";
    }
}

// NoQuarterState.java
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 String toString() {
        return "waiting for quarter";
    }
}

// HasQuarterState.java
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 String toString() {
        return "waiting for turn of crank";
    }
}

// WinnerState.java
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() {
        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());
            }
        }
    }
                          
    public String toString() {
        return "despensing two gumballs for your quarter, because YOU'RE A WINNER!";
    }
}

// GumballMachine.java
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;
        state = noQuarterState;
    }
    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();
    }
}

测试代码

//GumballMachineTestDrive.java
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);
    }
}

运行效果

Mighty Gumball, Inc. Java-enabled Standing Gumball Model #2004 Inventory: 10 gumballs Machine is waiting for quarter You inserted a quarter You turned... A gumball comes rolling out the slot... You inserted a quarter You turned... A gumball comes rolling out the slot... Mighty Gumball, Inc. Java-enabled Standing Gumball Model #2004 Inventory: 8 gumballs Machine is waiting for quarter You inserted a quarter You turned... A gumball comes rolling out the slot... You inserted a quarter You turned... A gumball comes rolling out the slot... Mighty Gumball, Inc. Java-enabled Standing Gumball Model #2004 Inventory: 6 gumballs Machine is waiting for quarter You inserted a quarter You turned... A gumball comes rolling out the slot... You inserted a quarter You turned... A gumball comes rolling out the slot... Mighty Gumball, Inc. Java-enabled Standing Gumball Model #2004 Inventory: 4 gumballs Machine is waiting for quarter You inserted a quarter You turned... A gumball comes rolling out the slot... You inserted a quarter You turned... A gumball comes rolling out the slot... Mighty Gumball, Inc. Java-enabled Standing Gumball Model #2004 Inventory: 2 gumballs Machine is waiting for quarter You inserted a quarter You turned... A gumball comes rolling out the slot... You inserted a quarter You turned... A gumball comes rolling out the slot... Oops, out of gumballs! Mighty Gumball, Inc. Java-enabled Standing Gumball Model #2004 Inventory: 0 gumballs Machine is sold out