带有数字常量的枚举类型

Question

我从遗留系统获取数据,其中某个字节字段是可能包含字母或数字的代码.我想将它映射到枚举,但我不知道如何处理数值.

public enum UsageCode {
    A ("Antique"),
    F ("Flood Damaged"),
    N ("New");
//  0 ("Unknown")  How to allow for value of "0"?

    private final String description;

    UsageCode(String description) {
        this.description = description;
    }

    public String getDescription() {
        return description;
    }
}

Answer 1

把它翻出来:

public enum UsageCode {
    ANTIQUE ('A'),
    FLOOD_DAMAGED ('F'),
    NEW ('N');
    UNKNOWN ('0')

    private static final Map<Character, UsageCode> charToEnum
            = new HashMap<Character, UsageCode>();

    static { // Initialize map from legacy code to enum constant
        for (UsageCode code : values())
        charToEnum.put(code.getCode(), code);
    }

    // Returns UsageCode for legacy character code, or null if code is invalid
    public static UsageCode fromLegacyCode(char code) {
        return charToEnum.get(code);
    }

    private final char code;

    UsageCode(char code) {
        this.code = code;
    }

    public char getCode() {
        return code;
    }
}

为了将输入的字符代码转换为枚举值,我添加了内部Map<Character, UsageCode>和静态转换方法.

示例改编自Effective Java 2nd Edition,Item 30.

Answer 2

您可以以其他方式执行此操作,具有有意义的常量并存储遗留值表示:

public enum UsageCode {

   ANTIQUE("A"),
   FLOOD_DAMAGED("F"),
   NEW("N"),
   UNKNOWN("0");

   private String legacy;

   private UsageCode(String legacy) {
      this.legacy = legacy;
   }

   public static UsageCode toUsageCode(String legacyOutput) {
      for(UsageCode code : values()) {
         if (code.legacy.equals(legacyOutput)) {
            return code;
         }
      }
      return null;
   }
}