State pattern

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State pattern

问题定义

假设我们需要制造一台糖果机，其操作如下:

Design_pattern_state_1

我们可以分清楚各个状态以及状态的转换，当做一个简单的状态机来实现：

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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();
		}
	}
 
	public 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();
	}
}
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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.ejectQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);

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

		System.out.println(gumballMachine);

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

		System.out.println(gumballMachine);
	}
}

当需要增加功能时，这份代码将十分难以修改和扩展，比如现在我们需要当曲柄被转动时，有10%的记录掉下两颗糖果。

更好的设计是：

  1. 定义一个state接口。在这个接口内，糖果机的每个动作都有一个对应的方法。
  2. 然后为机器的每个状态实现状态类。这些类将入则在对应状态下进行机器的行为。
  3. 将动作委托到状态类。

Design_pattern_state_2

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

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

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";
	}
}

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";
	}
}
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public class GumballMachine {
 
	State soldOutState;
	State noQuarterState;
	State hasQuarterState;
	State soldState;
 
	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);

		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 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();
	}
}
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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);
	}
}

定义状态模式

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。

Design_pattern_state_3

状态模式跟策略模式形式上很相似，但是，意图不同！

状态模式将一群行为封装在状态对象中，运行过程中当前状态在状态对象集合中游走改变，但是客户端对于状态对象完全不了解。状态模式一般是用来避免在context中放置许多条件判断的替代方案，以便于扩展。

而策略模式中，通常是客户主动指定所需要的策略对象，以满足所需要的行为。策略模式是除继承之外一种弹性替代方案，可以避免被父类的行为困住，我们可以通过组合不同的对象来改变行为。

解决扩展问题

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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();
	}
}
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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!";
	}
}
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package headfirst.state.gumballstatewinner;

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);
	}
}