1.應用場景

在軟體開發過程中，應用程式可能會根據不同的情況作出不同的處理。最直接的解決方案是將這些所有可能發生的情況全都考慮到。然後使用if… ellse語句來做狀態判斷來進行不同情況的處理。但是對複雜狀態的判斷就顯得“力不從心了”。隨著增加新的狀態或者修改一個狀體（if else(或switch case)語句的增多或者修改）可能會引起很大的修改，而程式的可讀性，擴充套件性也會變得很弱。維護也會很麻煩。那麼我就考慮只修改自身狀態的模式。

例子1：按鈕來控制一個電梯的狀態，一個電梯開們，關門，停，執行。每一種狀態改變，都有可能要根據其他狀態來更新處理。例如，開門狀體，你不能在執行的時候開門，而是在電梯定下後才能開門。

例子2：我們給一部手機打電話，就可能出現這幾種情況：使用者開機，使用者關機，使用者欠費停機，使用者消戶等。 所以當我們撥打這個號碼的時候：系統就要判斷，該使用者是否在開機且不忙狀態，又或者是關機，欠費等狀態。但不管是那種狀態我們都應給出對應的處理操作。

2. 概念

允許物件在內部狀態改變時改變它的行為，物件看起來好像修改了它所屬的類。

這個模式將狀態封裝成了獨立的類，並且將動作委託到代表當前狀態的物件上，我們知道行為會隨著內部狀態的改變而改變。

3.Class Diagram

Context（上下文）是一個類，它可以擁有一些內部狀態。

State介面定義了一個所有具體狀態的共同介面，任何狀態都實現這個相同的介面，這樣一來，狀態之間可以互相替換。

4. Implementation

糖果銷售機有多種狀態，每種狀態下銷售機有不同的行為，狀態可以發生轉移，使得銷售機的行為也發生改變。

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);
	}
}