【行为型模式】状态模式

一、状态模式概述

状态模式的定义：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。(对象行为型)

策略模式和状态模式是双胞胎，在出生时才分开。

策略模式是围绕可以互换的算法来创建成功业务的。
状态模式走的是更崇高的路，它通过改变对象内部的状态来帮助对象控制自己的行为。

状态模式的优缺点：
- 优点：
  - 1.可以将不同的状态隔离；
  - 2.每个状态都是一个单独的类；
  - 3.可以将各种状态的转换逻辑 , 分布到状态的子类中，减少相互依赖；
  - 4.增加新状态操作简单。
- 缺点：
  - 如果状态数量比较多，状态类的数量会增加，业务场景系统变得很复杂；
  - 如果业务中某个对象由几十上百个状态，就会很复杂，这时就需要对状态进行拆分处理。
适用场景：
- 一个对象存在多个状态，状态可以相互转换；
- 不同状态下，行为不同。

二、代码实现

状态模式主要包含三个角色：

环境（Context）角色：也称为上下文，它定义了客户感兴趣的接口，维护一个当前状态，并将与状态相关的操作委托给当前状态对象来处理；
抽象状态（State）角色：定义一个接口，用以封装环境对象中的特定状态所对应的行为；
具体状态（Concrete State）角色：实现抽象状态所对应的行为。

2.1 环境角色(GumballMachine)

package state.CandySolder;
//环境角色
public class GumballMachine {private State noQuarterState;private State hasQuarterState;private State soldState;private State soldOutState;private State state = soldOutState; // 糖果机默认状态为售罄状态int count = 0; // 糖果库存量public GumballMachine(int numberGumballs) {noQuarterState = new NoQuarterState(this);hasQuarterState = new HasQuarterState(this);soldState = new SoldState(this);soldOutState = new SoldOutState(this);count = numberGumballs;if (numberGumballs > 0) {state = noQuarterState; // 如果采购了糖果球（numberGumballs>0），则糖果机的状态为未投币状态}}// 投入钱币public void insertQuarter() {state.insertQuarter();}// 退出钱币public void ejectQuarter() {state.ejectQuarter();}// 扭转曲柄public void turnCrank() {state.turnCrank();state.dispense();}// 减少库存public void releaseBall() {if (count > 0) {System.out.println("一个糖果球正在出库");--count;} else {System.out.println("库存不足，一个糖果球无法出库");}}// 设置状态void setState(State state) {this.state = state;}//get，set方法public State getNoQuarterState() {return noQuarterState;}public State getSoldOutState() {return soldOutState;}public void setSoldOutState(State soldOutState) {this.soldOutState = soldOutState;}public int getCount() {return count;}public void setCount(int count) {this.count = count;}public void setNoQuarterState(State noQuarterState) {this.noQuarterState = noQuarterState;}public State getHasQuarterState() {return hasQuarterState;}public void setHasQuarterState(State hasQuarterState) {this.hasQuarterState = hasQuarterState;}public State getSoldState() {return soldState;}public void setSoldState(State soldState) {this.soldState = soldState;}}

2.2 抽象状态(State)

package state.CandySolder;
//抽象角色
public interface State {void insertQuarter(); // 投入硬币操作   void ejectQuarter(); // 退出硬币操作void turnCrank(); // 扭转曲柄操作void dispense(); // 发放糖果操作
}

2.3 具体状态(HasQuarterState、NoQuarterState、SoldOutState、SoldState)

package state.CandySolder;
//具体状态，已经投放钱币状态
public class HasQuarterState implements State {private GumballMachine gumballMachine;public HasQuarterState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {// TODO Auto-generated method stubSystem.out.println("您已经投入钱币！无须再次投入钱币！");}@Overridepublic void ejectQuarter() {// TODO Auto-generated method stubSystem.out.println("退款成功！");gumballMachine.setState(gumballMachine.getNoQuarterState()); // 状态流转}@Overridepublic void turnCrank() {// TODO Auto-generated method stubSystem.out.println("正在出货中，请稍等");gumballMachine.setState(gumballMachine.getSoldState()); // 状态流转}@Overridepublic void dispense() {// TODO Auto-generated method stubSystem.out.println("你还没有扭转曲柄，糖果不可以发放！");}}

package state.CandySolder;
//具体状态，没有投放钱币状态
public class NoQuarterState implements State {private GumballMachine gumballMachine;public NoQuarterState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {// TODO Auto-generated method stubSystem.out.println("投入钱币成功！");gumballMachine.setState(gumballMachine.getHasQuarterState()); // 状态流转}@Overridepublic void ejectQuarter() {// TODO Auto-generated method stubSystem.out.println("你还没有投入钱币，不能退回钱币！");}@Overridepublic void turnCrank() {// TODO Auto-generated method stubSystem.out.println("你还没有投入钱币，不能扭转曲柄！");}@Overridepublic void dispense() {// TODO Auto-generated method stubSystem.out.println("你还没有投入钱币，糖果不可以发放！");}}

package state.CandySolder;
//具体状态，糖果售空状态
public class SoldOutState implements State {private GumballMachine gumballMachine;public SoldOutState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {// TODO Auto-generated method stubSystem.out.println("糖果已经售罄。不能投入钱币");}@Overridepublic void ejectQuarter() {// TODO Auto-generated method stubSystem.out.println("退回钱币成功！");}@Overridepublic void turnCrank() {// TODO Auto-generated method stubSystem.out.println("糖果已经售罄。不能扭转曲柄！");}@Overridepublic void dispense() {// TODO Auto-generated method stubSystem.out.println("糖果已经售罄。糖果无法出售！");}}

package state.CandySolder;
//具体状态，糖果售卖状态
public class SoldState implements State {private GumballMachine gumballMachine;public SoldState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {// TODO Auto-generated method stubSystem.out.println("糖果正在出货中，请稍等。无须再次投入钱币！");}@Overridepublic void ejectQuarter() {// TODO Auto-generated method stubSystem.out.println("糖果正在出货中，请稍等。不能退回钱币！");}@Overridepublic void turnCrank() {// TODO Auto-generated method stubSystem.out.println("糖果正在出货中，请稍等。不需要再次扭转曲柄！");}@Overridepublic void dispense() {// TODO Auto-generated method stubif (gumballMachine.getCount() > 0) {System.out.println("糖果正在出货中，请稍等！");gumballMachine.releaseBall();gumballMachine.setState(gumballMachine.getNoQuarterState()); // 状态流转} else {System.out.println("糖果库存不足，无法出货！");gumballMachine.setState(gumballMachine.getSoldOutState()); // 状态流转}}}

2.4 main方法实现状态模式

package state.CandySolder;
//测试
public class Test {public static void main(String[] args) {System.out.println("-----向糖果机中放入1枚糖果-----");GumballMachine machine = new GumballMachine(1);System.out.println("-----第一次购买糖果-----");machine.insertQuarter();machine.ejectQuarter();machine.turnCrank();System.out.println("-----第二次购买糖果-----");machine.insertQuarter();machine.turnCrank();System.out.println("-----第三次购买糖果-----");machine.insertQuarter();machine.turnCrank();machine.ejectQuarter();}
}