介绍

有限状态机通常用于模拟序列逻辑，换句话说，就是用于代表和控制执行流程。

有限状态机所需条件：

一个物体只有固定的几种状态(例如交通灯只有绿灯、黄灯和红灯三个状态)。

有固定的变化顺序(例如交通灯只能绿灯->黄灯->红灯->绿灯循环)

物体同一时间点上只有一种状态。

例子

这里拿一个空调做例子，图是用了别人的，这里说明下下面三个状态的意思：

Off：空调关机

FanOnly：空调仅吹风

Cool：空调制冷

原始的解决办法

StateEnum

用枚举类定义三种状态

package org.example.origin;

public enum StateEnum {

POWER_OFF, FAN_ONLY, COOL

}

OriginAction

pressPowerOff() 表示点开关按钮，pressCool() 表示点击制冷按钮。

package org.example.origin;

public class OriginAction {

private StateEnum stateEnum;

public OriginAction() {

this.stateEnum = StateEnum.POWER_OFF;

}

public void pressPowerOff() {

// 关机->吹风

if (stateEnum == StateEnum.POWER_OFF) {

this.stateEnum = StateEnum.FAN_ONLY;

System.out.println("Power on succeed.");

} else {

// 吹风或制冷->关机

this.stateEnum = StateEnum.POWER_OFF;

System.out.println("Power off succeed.");

}

}

public void pressCool() {

if (stateEnum == StateEnum.POWER_OFF) {

System.out.println("Cool failed because power off.");

} else if (stateEnum == StateEnum.FAN_ONLY) {

// 吹风->制冷

this.stateEnum = StateEnum.COOL;

System.out.println("Open cool succeed.");

} else {

// 制冷->吹风

this.stateEnum = StateEnum.FAN_ONLY;

System.out.println("Close cool succeed");

}

}

}

OriginTest

编写测试类

package org.example.origin;

public class OriginTest {

public static void main(String[] args) {

OriginAction originAction = new OriginAction();

originAction.pressCool();

originAction.pressPowerOff();

originAction.pressCool();

originAction.pressCool();

originAction.pressPowerOff();

}

}

执行结果：

Cool failed because power off.

Power on succeed.

Open cool succeed.

Close cool succeed

Power off succeed.

使用有限状态机解决

FiniteStateMachineEnum

这里使用了 Java 的枚举类，确保了类型约束。

package org.example.state;

public enum FiniteStateMachineEnum {

POWER_OFF {

// 关机->吹风

@Override

public FiniteStateMachineEnum pressPowerOff() {

System.out.println("Power on succeed.");

return FiniteStateMachineEnum.FAN_ONLY;

}

// 无效

@Override

public FiniteStateMachineEnum pressCool() {

System.out.println("Cool failed because power off.");

return FiniteStateMachineEnum.POWER_OFF;

}

},

FAN_ONLY {

// 吹风->关机

@Override

public FiniteStateMachineEnum pressPowerOff() {

System.out.println("Power off succeed.");

return FiniteStateMachineEnum.POWER_OFF;

}

// 吹风->制冷

@Override

public FiniteStateMachineEnum pressCool() {

System.out.println("Open cool succeed.");

return FiniteStateMachineEnum.COOL;

}

},

COOL {

// 制冷->关机

@Override

public FiniteStateMachineEnum pressPowerOff() {

System.out.println("Power off succeed.");

return FiniteStateMachineEnum.POWER_OFF;

}

// 制冷->吹风

@Override

public FiniteStateMachineEnum pressCool() {

System.out.println("Close cool succeed");

return FiniteStateMachineEnum.FAN_ONLY;

}

};

public abstract FiniteStateMachineEnum pressPowerOff();

public abstract FiniteStateMachineEnum pressCool();

}

FiniteStateMachineTest

编写测试类

package org.example.state;

public class FiniteStateMachineTest {

public static void main(String[] args) {

FiniteStateMachineEnum stateEnum = FiniteStateMachineEnum.POWER_OFF;

stateEnum.pressCool()

.pressPowerOff()

.pressCool()

.pressCool()

.pressPowerOff();

}

}

执行结果：

Cool failed because power off.

Power on succeed.

Open cool succeed.

Close cool succeed

Power off succeed.

总结

两种方法的运行结果是一样的，但是有限状态机的写法更加简洁明了。传统方法在简单的条件下还可以应付，但是如果面对复杂的执行逻辑顺序，只有通过有限状态机来解决了，而且代码更加简洁明了，易于维护。