Java设计模式(十二)：状态设计模式

1.应用场景

在软件开发过程中，应用程序可能会根据不同的情况作出不同的处理。最直接的解决方案是将这些所有可能发生的情况全都考虑到。然后使用if… ellse语句来做状态判断来进行不同情况的处理。但是对复杂状态的判断就显得“力不从心了”。随着增加新的状态或者修改一个状体（if else(或switch case)语句的增多或者修改）可能会引起很大的修改，而程序的可读性，扩展性也会变得很弱。维护也会很麻烦。那么我就考虑只修改自身状态的模式。

例子1：按钮来控制一个电梯的状态，一个电梯开们，关门，停，运行。每一种状态改变，都有可能要根据其他状态来更新处理。例如，开门状体，你不能在运行的时候开门，而是在电梯定下后才能开门。

例子2：我们给一部手机打电话，就可能出现这几种情况：用户开机，用户关机，用户欠费停机，用户消户等。 所以当我们拨打这个号码的时候：系统就要判断，该用户是否在开机且不忙状态，又或者是关机，欠费等状态。但不管是那种状态我们都应给出对应的处理操作。

2. 概念

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所属的类。

这个模式将状态封装成了独立的类，并且将动作委托到代表当前状态的对象上，我们知道行为会随着内部状态的改变而改变。

3.Class Diagram

Context（上下文）是一个类，它可以拥有一些内部状态。

State接口定义了一个所有具体状态的共同接口，任何状态都实现这个相同的接口，这样一来，状态之间可以互相替换。

4. Implementation

糖果销售机有多种状态，每种状态下销售机有不同的行为，状态可以发生转移，使得销售机的行为也发生改变。

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

    public void refill();
}

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";
	}
}

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";
	}
}

public class NoQuarterStete implements State {
    private GumballMachine gumballMachine;

    public NoQuarterStete(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    @Override
    public void insertQuarter() {
        System.out.println("You inserted a quarter");
        gumballMachine.setState(gumballMachine.hasQuarterState);
    }

    @Override
    public void ejectQuarter() {
        System.out.println("You haven't inserted a quarter.");
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned, but there's no quarter. ");
    }

    @Override
    public void dispense() {
        System.out.println("you need to pay first");
    }

    @Override
    public void refill() {
    }
}

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";
	}
}

public class HasQuarterState implements State {
	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...");
		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";
	}
}

public class GumballMachine {
 
	State soldOutState;
	State noQuarterState;
	State hasQuarterState;
	State soldState;
 
	State state;
	int count = 0;
 
	public GumballMachine(int numberGumballs) {
		soldOutState = new SoldOutState(this);
		noQuarterState = new NoQuarterState(this);
		hasQuarterState = new HasQuarterState(this);
		soldState = new SoldState(this);

		this.count = numberGumballs;
 		if (numberGumballs > 0) {
			state = noQuarterState;
		} else {
			state = soldOutState;
		}
	}
 
	public void insertQuarter() {
		state.insertQuarter();
	}
 
	public void ejectQuarter() {
		state.ejectQuarter();
	}
 
	public void turnCrank() {
		state.turnCrank();
		state.dispense();
	}
 
	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; it's new count is: " + this.count);
		state.refill();
	}

	void setState(State state) {
		this.state = state;
	}
    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 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();
	}
}

public class GumballMachineTestDrive {

	public static void main(String[] args) {
		GumballMachine gumballMachine = new GumballMachine(2);

		System.out.println(gumballMachine);

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

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		
		gumballMachine.refill(5);
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);
	}
}