原文地址：https://www.inlighting.org/archives/finite-state-machine-in-java

介紹

有限狀態(tài)機(jī)通常用于模擬序列邏輯，換句話說，就是用于代表和控制執(zhí)行流程。

有限狀態(tài)機(jī)所需條件：

• 一個(gè)物體只有固定的幾種狀態(tài)（例如交通燈只有綠燈、黃燈和紅燈三個(gè)狀態(tài)）四苇。
• 有固定的變化順序（例如交通燈只能綠燈->黃燈->紅燈->綠燈循環(huán)）
• 物體同一時(shí)間點(diǎn)上只有一種狀態(tài)。

例子

這里拿一個(gè)空調(diào)做例子，圖是用了別人的臀栈，這里說明下下面三個(gè)狀態(tài)的意思：

Off：空調(diào)關(guān)機(jī)

FanOnly：空調(diào)僅吹風(fēng)

Cool：空調(diào)制冷

image

原始的解決辦法

StateEnum

用枚舉類定義三種狀態(tài)

package org.example.origin;

public enum StateEnum {
    POWER_OFF, FAN_ONLY, COOL
}

OriginAction

pressPowerOff() 表示點(diǎn)開關(guān)按鈕，pressCool() 表示點(diǎn)擊制冷按鈕挠乳。

package org.example.origin;

public class OriginAction {
    private StateEnum stateEnum;

    public OriginAction() {
        this.stateEnum = StateEnum.POWER_OFF;
    }

    public void pressPowerOff() {
        // 關(guān)機(jī)->吹風(fēng)
        if (stateEnum == StateEnum.POWER_OFF) {
            this.stateEnum = StateEnum.FAN_ONLY;
            System.out.println("Power on succeed.");
        } else {
            // 吹風(fēng)或制冷->關(guān)機(jī)
            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) {
            // 吹風(fēng)->制冷
            this.stateEnum = StateEnum.COOL;
            System.out.println("Open cool succeed.");
        } else {
            // 制冷->吹風(fēng)
            this.stateEnum = StateEnum.FAN_ONLY;
            System.out.println("Close cool succeed");
        }
    }
}

OriginTest

編寫測(cè)試類

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();
    }
}

執(zhí)行結(jié)果：

Cool failed because power off.
Power on succeed.
Open cool succeed.
Close cool succeed
Power off succeed.

使用有限狀態(tài)機(jī)解決

FiniteStateMachineEnum

這里使用了 Java 的枚舉類权薯，確保了類型約束姑躲。

package org.example.state;

public enum FiniteStateMachineEnum {

    POWER_OFF {
        // 關(guān)機(jī)->吹風(fēng)
        @Override
        public FiniteStateMachineEnum pressPowerOff() {
            System.out.println("Power on succeed.");
            return FiniteStateMachineEnum.FAN_ONLY;
        }

        // 無效
        @Override
        public FiniteStateMachineEnum pressCool() {
            System.out.println("Cool failed because power off.");
            return FiniteStateMachineEnum.POWER_OFF;
        }
    },

    FAN_ONLY {
        // 吹風(fēng)->關(guān)機(jī)
        @Override
        public FiniteStateMachineEnum pressPowerOff() {
            System.out.println("Power off succeed.");
            return FiniteStateMachineEnum.POWER_OFF;
        }

        // 吹風(fēng)->制冷
        @Override
        public FiniteStateMachineEnum pressCool() {
            System.out.println("Open cool succeed.");
            return FiniteStateMachineEnum.COOL;
        }
    },

    COOL {
        // 制冷->關(guān)機(jī)
        @Override
        public FiniteStateMachineEnum pressPowerOff() {
            System.out.println("Power off succeed.");
            return FiniteStateMachineEnum.POWER_OFF;
        }

        // 制冷->吹風(fēng)
        @Override
        public FiniteStateMachineEnum pressCool() {
            System.out.println("Close cool succeed");
            return FiniteStateMachineEnum.FAN_ONLY;
        }
    };

    public abstract FiniteStateMachineEnum pressPowerOff();

    public abstract FiniteStateMachineEnum pressCool();
}

FiniteStateMachineTest

編寫測(cè)試類

package org.example.state;

public class FiniteStateMachineTest {
    public static void main(String[] args) {
        FiniteStateMachineEnum stateEnum = FiniteStateMachineEnum.POWER_OFF;
        stateEnum.pressCool()
                .pressPowerOff()
                .pressCool()
                .pressCool()
                .pressPowerOff();
    }
}

執(zhí)行結(jié)果：

Cool failed because power off.
Power on succeed.
Open cool succeed.
Close cool succeed
Power off succeed.

總結(jié)

兩種方法的運(yùn)行結(jié)果是一樣的，但是有限狀態(tài)機(jī)的寫法更加簡(jiǎn)潔明了崭闲。傳統(tǒng)方法在簡(jiǎn)單的條件下還可以應(yīng)付肋联，但是如果面對(duì)復(fù)雜的執(zhí)行邏輯順序，只有通過有限狀態(tài)機(jī)來解決了刁俭，而且代碼更加簡(jiǎn)潔明了橄仍，易于維護(hù)。