定义：当一个对象的内部状态发生改变时，它的行为也随着改变，就好像换了一个类一样。

使用场景：同一个上下文中，某个对象有不同的状态，并且不同的状态行为不一样

类图：

代码样例：

package headfirst.state.gumballstate;

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

package headfirst.state.gumballstate;

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";
	}
}

package headfirst.state.gumballstate;

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";
	}
}

package headfirst.state.gumballstate;

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";
	}
}

package headfirst.state.gumballstate;

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();
	}
}

package headfirst.state.gumballstate;

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);
	}
}

优点：1）实现不同状态的行为的解耦

缺点：1）一个状态对应一个类，状态太多会导致产生很多的类

类似的设计模式：1）策略模式：状态模式强调同一个上下文状态的变化，策略模式下不同的实现一般不会应用在同一上下文中

状态模式——Head First Design Patterns