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Java 使用 Enum 小記

發表於 更新於 分類於 Disqus：

平常不怎麼使用 Enum ，非常地不熟悉，今天剛好用到，順便記錄一下。

簡單使用

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public enum Action {
    STOP, RIGHT, LEFT, UP, DOWN
}

這邊簡單可以看到

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public static void play(int action) {
    switch(action) {
        case Action.STOP:
            System.out.println("播放停止動畫");
            break;
        略...
        default:
            System.out.println("不支援此動作");
    }
}

以前抄抄範例，這邊會發現int action
怎麼會跑出 int 出來？

其實 Enum 反編譯出來，會跟原本 Enum 出來不一樣
不知道這算不算是 Java 語法糖?

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略...
static {
    STOP = new Action("STOP", 0);
    RIGHT = new Action("RIGHT", 1);
    LEFT = new Action("LEFT", 2);
    UP = new Action("UP", 3);
    DOWN = new Action("DOWN", 4);
    略...
}

詳細內容可以看:使用 enum 列舉常數

自定一些內容

我們公司使用不單是這樣
可以設定出

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public enum Car {
    MAX_CAR(100,"最高級車"), NORM_CAR(5,"中級車"), MIN_CAR(1,"低級車"); 
    
    private int price;
    private String desc;
    
    private Car(int price, String desc) {
        this.price = price;
        this.desc = desc;
    }

    public int price() {
        return price;
    }
    
    public String desc() {
        return desc;
    }
    
    public static void main(String[] args) {
        for(Car car : Car.values()) {
            System.out.printf("Car(%s, %d, %s)%n",
                  car , car.price() , car.desc());
        }
    }
}

這邊看到會對印到 MAX_CAR(100,"最高級車"), NORM_CAR(5,"中級車"), MIN_CAR(1,"低級車");

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private Car(int price, String desc) {
    this.price = price;
    this.desc = desc;
}

其他詳細可看:進階 enum 運用

取出列表資料(values)

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for(Car car: Car.values())
    System.out.println(car.name());
}

.values() 可以印出列表出來

那有沒有find 呢?
目前看應該只能用迴圈滾…

轉成陣列方法

You can do (pre-Java 8):

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List<Enum> enumValues = Arrays.asList(Enum.values());

or
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List<Enum> enumValues = new ArrayList<Enum>(EnumSet.allOf(Enum.class));

Using Java 8 features, you can map each constant to its name:
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List<String> enumNames = Stream.of(Enum.values())
                               .map(Enum::name)
                               .collect(Collectors.toList());

Java: Check if enum contains a given string? - Stack Overflow

This should do it:

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public static boolean contains(String test) {

    for (Choice c : Choice.values()) {
        if (c.name().equals(test)) {
            return true;
        }
    }

    return false;
}

This way means you do not have to worry about adding additional enum values later, they are > all checked.

Edit: If the enum is very large you could stick the values in a HashSet:

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public static HashSet<String> getEnums() {

  HashSet<String> values = new HashSet<String>();

  for (Choice c : Choice.values()) {
      values.add(c.name());
  }

  return values;
}

Then you can just do: values.contains(“your string”) which returns true or false.

values() 神奇 method 實作方法

參考: Java Enum - javatpoint
原理 static 放 $VALUES 陣列變數，使用values() 會傳 $VALUES 陣列變數 ，就可以 foreach 比較。

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final class Season extends Enum  
{  
    public static Season[] values()  
    {  
        return (Season[])$VALUES.clone();  
    }  
    public static Season valueOf(String s)  
    {  
        return (Season)Enum.valueOf(Season, s);  
    }  
    private Season(String s, int i, int j)  
    {  
        super(s, i);  
        value = j;  
    }  
    public static final Season WINTER;  
    public static final Season SUMMER;  
    private int value;  
    private static final Season $VALUES[];  
    static   
    {  
        WINTER = new Season("WINTER", 0, 10);  
        SUMMER = new Season("SUMMER", 1, 20);  
        $VALUES = (new Season[] {  
            WINTER, SUMMER  
        });  
    }  
}

其他之後有遇到再補

2020-06-18

特別載入環境變數方法

最近做 Enum 發現測試環境跟正式環境設定的值會不一樣，所以找一下有什麼方法可以解決這個問題。
目前這邊還沒測試能不能這樣用。

java property file as enum - Stack Overflow

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public enum Constants {
    PROP1,
    PROP2;

    private static final String PATH            = "/constants.properties";

    private static final Logger logger          = LoggerFactory.getLogger(Constants.class);

    private static Properties   properties;

    private String          value;

    private void init() {
        if (properties == null) {
            properties = new Properties();
            try {
                properties.load(Constants.class.getResourceAsStream(PATH));
            }
            catch (Exception e) {
                logger.error("Unable to load " + PATH + " file from classpath.", e);
                System.exit(1);
            }
        }
        value = (String) properties.get(this.toString());
    }

    public String getValue() {
        if (value == null) {
            init();
        }
        return value;
    }

}

enum 裡面帶 method 方法

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//1.提供当前枚举类的对象，多个对象之间用逗号隔开，末尾对象分号结束
  SPRING ("春天","春暖花开"){
       @Override
       public void show() {
           System.out.println("spring!!!");
       }
   },
  SUMMER("夏天","夏日炎炎"){
      @Override
      public void show() {
          System.out.println("summer!!!");
      }
  },
  AUTUMN ("秋天","秋高气爽"){
      @Override
      public void show() {
          System.out.println("autumn!!!");
      }
  },
  WINTER ("冬天","冰天雪地"){
      @Override
      public void show() {
          super.show();
          System.out.println("winter!!!");
      }
  };

參考: java枚举和注解 - 知乎

實用範例

方便以後直接找 sample code 來用，畢竟真的不常寫，很容易要想怎麼寫。
最佳範例: 菜鳥工程師 肉豬: Java 列舉(Enum)範例
還是推薦看裡面內容。

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public enum WeekDay {

    SUN("Sunday","星期日", 0),
    MON("Monday","星期一", 1),
    TUE("Tuesday","星期二", 2),
    WED("Wednesday","星期三", 3),
    THU("Thursday","星期四", 4), 
    FRI("Friday","星期五", 5),
    SAT("Saturday","星期六", 6);

    WeekDay(String day, String chinese, int code){
        this.day = day;
        this.chinese = chinese;
        this.code = code;
    }

    private final String day;
    private final String chinese;
    private int code;

    public static WeekDay getWeek(int i){
        for(WeekDay weekDay : values()){
            if(weekDay.getCode() == i) {
                return weekDay;
            }
        }
        return null;
    }

    public String getDay(){
        return this.day;
    }
    
    public String getChinese() {
        return this.chinese;
    }

    public int getCode() {
        return this.code;
    }
  
}