状态模式（State）
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。
栗子
现在有一台糖果机，它的状态（挺复杂的）：
- 没有 25 分钱 -> 投入 25 分钱 -> 有 25 分钱
- 有 25 分钱 -> 转动曲柄 -> 售出糖果（数量不为0） | 糖果售罄（数量为0）
- 有 25 分钱 -> 退钱按钮 -> 退出 25 分钱
- 售出糖果 -> 没有 25 分钱
从上面的状态实现代码的步骤：
- 找出所有状态 -> 共四种：没有 25 分钱、有 25 分钱、售出糖果、糖果售罄。
- 创建一个持有当前状态的实例变量 state。
- 写出所有可能发生的动作判断。
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| class GumballMachine {
private final static int SOLD_OUT = 0;
private final static int NO_QUARTER = 1;
private final static int HAS_QUARTER = 2;
private final static int SOLD = 3;
private int state = SOLD_OUT;
private int count = 0;
public GumballMachine(int count) {
this.count = count;
if (count > 0) {
state = NO_QUARTER;
}
}
public void insertQuarter() {
if (state == HAS_QUARTER) {
System.out.println("你已经投过25分钱了，请不要重复投币");
} else if (state == NO_QUARTER) {
System.out.println("投进25分钱");
state = HAS_QUARTER;
} else if (state == SOLD_OUT) {
System.out.println("没有糖果了，不要投币");
} else if (state == SOLD) {
System.out.println("请稍等，正在出糖果");
}
}
public void ejectQuarter() {
if (state == HAS_QUARTER) {
System.out.println("已退还25分钱");
state = NO_QUARTER;
} else if (state == NO_QUARTER) {
System.out.println("你还没投币呢");
} else if (state == SOLD_OUT) {
System.out.println("没有糖果时无法投币，不要骗钱");
} else if (state == SOLD) {
System.out.println("你已经转动曲柄了，无法退钱");
}
}
public void turnCrank() {
if (state == HAS_QUARTER) {
System.out.println("你转动了曲柄");
state = SOLD;
dispense();
} else if (state == NO_QUARTER) {
System.out.println("你还没投币呢");
} else if (state == SOLD_OUT) {
System.out.println("已经没有糖果了");
} else if (state == SOLD) {
System.out.println("转几次都只能拿一次");
}
}
public void dispense() {
if (state == SOLD) {
System.out.println("糖果已发出");
count--;
if (count == 0) {
System.out.println("已经没有糖果喽");
state = SOLD_OUT;
} else {
state = NO_QUARTER;
}
} else if (state == NO_QUARTER) {
System.out.println("你还没投币呢");
} else if (state == SOLD_OUT) {
System.out.println("没有糖果发");
} else if (state == SOLD) {
System.out.println("没有糖果发");
}
}
}
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多么缜密的判断，基本上没什么漏洞了。
新需求
现在产品经理提需求来了：当曲柄被转动时，有 10% 的几率掉出两个糖果。
看回上面缜密的代码，是不是有点无从入手？这个需求真是要了命了……如果需求再有变更，上面的代码基本上就得推倒重来了，因为代码逻辑判断太复杂。
满足需求
我们可以把这四个状态当作一个实体，比如糖果机当前处在没投币的状态，那么在该状态下，可以通过投币动作使糖果机转移到投了币状态，其他状态也是这样，基本上可以把 if else 这些判断语句分离出来。
定义状态接口：
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| interface State {
void insertQuarter();
void ejectQuarter();
void turnCrank();
void dispense();
}
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然后每种状态都实现该接口（先看总体）：
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| class SoldState implements State
class SoldOutState implements State
class NoQuarterState implements State
class HasQuarterState implements State
class WinnerState implements State
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还有新的糖果机（还没添加中奖状态）：
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| class GumballMachine {
private State soldState;
private State soldOutState;
private State noQuarterState;
private State hasQuarterState;
private State state = soldState;
private int count = 0;
public GumballMachine(int count) {
soldState = new SoldState(this);
soldOutState = new SoldOutState(this);
noQuarterState = new NoQuarterState(this);
hasQuarterState = new HasQuarterState(this);
this.count = count;
if (count > 0) {
state = noQuarterState;
}
}
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("糖果已发出");
if (count != 0) {
count--;
}
}
public State getState() {
return state;
}
public int getCount() {
return count;
}
public State getSoldState() {
return soldState;
}
public State getSoldOutState() {
return soldOutState;
}
public State getNoQuarterState() {
return noQuarterState;
}
public State getHasQuarterState() {
return hasQuarterState;
}
}
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分别实现的状态类
没投币的状态：
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| class NoQuarterState implements State {
private GumballMachine gumballMachine;
public NoQuarterState(GumballMachine gumballMachine) {
this.gumballMachine = gumballMachine;
}
@Override
public void insertQuarter() {
System.out.println("投进25分钱");
gumballMachine.setState(gumballMachine.getHasQuarterState());
}
@Override
public void ejectQuarter() {
System.out.println("你还没投币呢，怎么退钱");
}
@Override
public void turnCrank() {
System.out.println("你转动了曲柄，但你还没投币呢");
}
@Override
public void dispense() {
System.out.println("你要先投币");
}
}
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投了币的状态：
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| class HasQuarterState implements State {
private GumballMachine gumballMachine;
public HasQuarterState(GumballMachine gumballMachine) {
this.gumballMachine = gumballMachine;
}
@Override
public void insertQuarter() {
System.out.println("你已经投过25分钱了，请不要重复投币");
}
@Override
public void ejectQuarter() {
System.out.println("已退还25分钱");
gumballMachine.setState(gumballMachine.getNoQuarterState());
}
@Override
public void turnCrank() {
System.out.println("你转动了曲柄");
gumballMachine.setState(gumballMachine.getSoldState());
}
@Override
public void dispense() {
System.out.println("没有糖果发");
}
}
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售出糖果的状态：
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| class SoldState implements State {
private GumballMachine gumballMachine;
public SoldState(GumballMachine gumballMachine) {
this.gumballMachine = gumballMachine;
}
@Override
public void insertQuarter() {
System.out.println("请稍等，正在出糖果");
}
@Override
public void ejectQuarter() {
System.out.println("你已经转动曲柄了，无法退钱");
}
@Override
public void turnCrank() {
System.out.println("转几次都只能拿一次");
}
@Override
public void dispense() {
gumballMachine.releaseBall();
if (gumballMachine.getCount() > 0) {
gumballMachine.setState(gumballMachine.getNoQuarterState());
} else {
System.out.println("已经没有糖果喽");
gumballMachine.setState(gumballMachine.getSoldOutState());
}
}
}
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糖果售罄的状态：
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| class SoldOutState implements State {
private GumballMachine gumballMachine;
public SoldOutState(GumballMachine gumballMachine) {
this.gumballMachine = gumballMachine;
}
@Override
public void insertQuarter() {
System.out.println("没有糖果了，不要投币");
}
@Override
public void ejectQuarter() {
System.out.println("没有糖果时无法投币，不要骗钱");
}
@Override
public void turnCrank() {
System.out.println("你转动了曲柄，但是已经没有糖果了");
}
@Override
public void dispense() {
System.out.println("没有糖果发");
}
}
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新需求中的中奖状态：
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| class WinnerState implements State {
private GumballMachine gumballMachine;
public WinnerState(GumballMachine gumballMachine) {
this.gumballMachine = gumballMachine;
}
@Override
public void insertQuarter() {
System.out.println("请稍等，正在出糖果");
}
@Override
public void ejectQuarter() {
System.out.println("你已经转动曲柄了，无法退钱");
}
@Override
public void turnCrank() {
System.out.println("转几次都只能拿一次");
}
@Override
public void dispense() {
System.out.println("恭喜！你拿到了两颗糖");
gumballMachine.releaseBall();
if (gumballMachine.getCount() == 0) {
gumballMachine.setState(gumballMachine.getSoldOutState());
} else {
gumballMachine.releaseBall();
if (gumballMachine.getCount() > 0) {
gumballMachine.setState(gumballMachine.getNoQuarterState());
} else {
System.out.println("已经没有糖果喽");
gumballMachine.setState(gumballMachine.getSoldOutState());
}
}
}
}
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需求还没解决完
上面的代码只是解决了状态的问题，但还没有完全满足需求，10% 的随机中奖还没写。（其实也没什么难度了）
给糖果机加上中奖状态：
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| class GumballMachine {
private State winnerState;
public GumballMachine(String location, int count) {
winnerState = new WinnerState(this);
}
public State getWinnerState() {
return winnerState;
}
}
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因为只有在投了币的情况下才能转动曲柄 -> 发糖 | 中奖，所以要在投了币的状态加入这 10% 随机：
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| class HasQuarterState implements State {
@Override
public void turnCrank() {
System.out.println("你转动了曲柄");
Random randomWinner = new Random(System.currentTimeMillis());
int winner = randomWinner.nextInt(10);
if (winner == 0 && gumballMachine.getCount() > 1) {
gumballMachine.setState(gumballMachine.getWinnerState());
} else {
gumballMachine.setState(gumballMachine.getSoldState());
}
}
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