設計模式——狀態模式

描述：

容許對象在內部狀態改變時改變它的行爲，對象看起來好像修改了它的類。

主要解決的是當控制一個對象狀態轉換的條件表達式過於複雜時的狀況。把狀態的判斷邏輯轉移到表示不一樣的一系列類當中，能夠把複雜的邏輯判斷簡單化。

一般應用在有好多狀態的流程中。

 

類圖：

 

如下程序模擬糖果機器投幣取糖果的狀態流程。

1.定義狀態結構

package net.dp.state.gumballstate;

public interface State {

	public void insertQuarter();

	public void ejectQuarter();

	public void turnCrank();

	public void dispense();
}

　　

2.沒有投幣狀態類

package net.dp.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";
	}
}

　　

3.已經投幣狀態

package net.dp.state.gumballstate;

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";
	}
}

　　

4.售罄狀態

package net.dp.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";
	}
}

　　

5.銷售

package net.dp.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";
	}
}

　　

6.聚合類

package net.dp.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();
	}
}

　　

7.測試

package net.dp.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);
	}
}