设计模式 - 状态模式-编程知识

一. 前言

二. 实现

一. 前言

状态模式（State Pattern）：它主要用来解决对象在多种状态转换时，需要对外输出不同的行为的问题。状态和行为是一一对应的，状态之间可以相互转换。当一个对象的内在状态改变时，允许改变其行为，这个对象看起来像是改变了其类。

状态模式和策略模式区别
1、策略模式封装了一组行为或者算法，它允许Client在运行时动态的切换；状态模式是帮助一个类在不同的状态下显示不同的行为，依赖于内部的状态；
2、策略模式不持有Context的引用，而是被Context所使用；状态模式的每个状态都持有Context的引用，从而在Context中实现状态的转移；
3、从理论上说，策略模式定义对象应该“怎么做”；状态模式定义了对象“是什么”，“什么时候做”。
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。

二. 实现

Context 类为环境角色, 用于维护State 实例,这个实例定义当前状态（聚合State）。
State 是抽象状态角色,定义一个接口封装与Context 的一个特点接口相关行为。
ConcreteState 具体的状态角色，每个子类实现一个与Context 的一个状态相关行为。

案例：糖果销售机有多种状态，每种状态下销售机有不同的行为，状态可以发生转移，使得销售机的行为也发生改变。

public interface State {/*** 投入 25 分钱*/void insertQuarter();/*** 退回 25 分钱*/void ejectQuarter();/*** 转动曲柄*/void turnCrank();/*** 发放糖果*/void dispense();
}

public class HasQuarterState implements State {private GumballMachine gumballMachine;public HasQuarterState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {System.out.println("You can't insert another quarter");}@Overridepublic void ejectQuarter() {System.out.println("Quarter returned");gumballMachine.setState(gumballMachine.getNoQuarterState());}@Overridepublic void turnCrank() {System.out.println("You turned...");gumballMachine.setState(gumballMachine.getSoldState());}@Overridepublic void dispense() {System.out.println("No gumball dispensed");}
}

public class NoQuarterState implements State {GumballMachine gumballMachine;public NoQuarterState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {System.out.println("You insert a quarter");gumballMachine.setState(gumballMachine.getHasQuarterState());}@Overridepublic void ejectQuarter() {System.out.println("You haven't insert a quarter");}@Overridepublic void turnCrank() {System.out.println("You turned, but there's no quarter");}@Overridepublic void dispense() {System.out.println("You need to pay first");}
}

public class SoldOutState implements State {GumballMachine gumballMachine;public SoldOutState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {System.out.println("You can't insert a quarter, the machine is sold out");}@Overridepublic void ejectQuarter() {System.out.println("You can't eject, you haven't inserted a quarter yet");}@Overridepublic void turnCrank() {System.out.println("You turned, but there are no gumballs");}@Overridepublic void dispense() {System.out.println("No gumball dispensed");}
}

public class SoldState implements State {GumballMachine gumballMachine;public SoldState(GumballMachine gumballMachine) {this.gumballMachine = gumballMachine;}@Overridepublic void insertQuarter() {System.out.println("Please wait, we're already giving you a gumball");}@Overridepublic void ejectQuarter() {System.out.println("Sorry, you already turned the crank");}@Overridepublic void turnCrank() {System.out.println("Turning twice doesn't get you another gumball!");}@Overridepublic 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 class GumballMachine {private State soldOutState;private State noQuarterState;private State hasQuarterState;private State soldState;private State state;private int count = 0;public GumballMachine(int numberGumballs) {count = numberGumballs;soldOutState = new SoldOutState(this);noQuarterState = new NoQuarterState(this);hasQuarterState = new HasQuarterState(this);soldState = new SoldState(this);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();}public void setState(State state) {this.state = state;}public void releaseBall() {System.out.println("A gumball comes rolling out the slot...");if (count != 0) {count -= 1;}}public State getSoldOutState() {return soldOutState;}public State getNoQuarterState() {return noQuarterState;}public State getHasQuarterState() {return hasQuarterState;}public State getSoldState() {return soldState;}public int getCount() {return count;}
}

public class Client {public static void main(String[] args) {GumballMachine gumballMachine = new GumballMachine(5);gumballMachine.insertQuarter();gumballMachine.turnCrank();gumballMachine.insertQuarter();gumballMachine.ejectQuarter();gumballMachine.turnCrank();gumballMachine.insertQuarter();gumballMachine.turnCrank();gumballMachine.insertQuarter();gumballMachine.turnCrank();gumballMachine.ejectQuarter();gumballMachine.insertQuarter();gumballMachine.insertQuarter();gumballMachine.turnCrank();gumballMachine.insertQuarter();gumballMachine.turnCrank();gumballMachine.insertQuarter();gumballMachine.turnCrank();}
}

You insert a quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
Quarter returned
You turned, but there's no quarter
You need to pay first
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You haven't insert a quarter
You insert a quarter
You can't insert another quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
You turned...
A gumball comes rolling out the slot...
Oops, out of gumballs
You can't insert a quarter, the machine is sold out
You turned, but there are no gumballs
No gumball dispensed