Mapping inside an enum

Question

I have an enum declared like this

 public enum KDErrors {

  KDR_280(280, "Blue"),
  KDR_281(281, "Red"),
  KDR_282(282, "Green"),
  KDR_284(284, "Yellow");

  private final int code;
  private final String color;

  private KDErrors(int code, String color) {
    this.code = code;
    this.color = color;
  }

  public static String getColorByCode(int colorCode) {

return ???
  }
}

I want to get the value by passing the code, how can I make it work?

If you are not using Enum instance directly anywhere, just declare a static map and add key-values as code-color and skip the entire enum creation. — Jimmy, Jul 13 '16 at 14:57
I think it is not a soublicate. OP do not want the Enum, OP wants an other Attribute from the enum — Jens, Jul 13 '16 at 15:02

score 2 · Accepted Answer · answered Jul 13 '16 at 14:50

Change code to:

public enum KDErrors {

    KDR_280(280, "Blue"), KDR_281(281, "Red"), KDR_282(282, "Green"), KDR_284(284, "Yellow");

    private final int code;

    private final String color;

    private KDErrors(int code, String color) {
        this.code = code;
        this.color = color;
    }

    public int getCode() {
        return code;
    }

    public String getColor() {
        return color;
    }

    public static String getColorByCode(int colorCode) {
        for (KDErrors error : KDErrors.values()) {
            if (error.getCode() == colorCode)
                return error.getColor();
        }
        return null;
    }

}

Abubakkar · Answer 2 · 2016-07-13T14:54:10.020

1

You could use a reverse lookup map of either you enum or code

Below is an example using reverse look up for enum based on code :

public enum KDErrors {

  private static Map<Integer, KDErrors> reverseLookUp = new HashMap<>();

  static{
    for (KDErrors error : KDErrors.values()) {
      reverseLookUp.put(error.code, error);
    }
  }
  //you method would look like
  public static String getColorByCode(int colorCode) {
    if(reverseLookUp.get(colorCode) == null)
      return null;
    else 
      return reverseLookUp.get(colorCode).color;
  }
}

edited Jul 13 '16 at 14:54

answered Jul 13 '16 at 14:51

Abubakkar

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1

This will throw a `NullPointerException` if the color code is not found in the map. You should do `Map` or check if the return value of `get` is `null` before accessing a field on it. – Mad Physicist Jul 13 '16 at 14:53
You don't need to call `get` twice. Do something like `err = reverseLookUp.get(colorCode); return err == null ? null : err.color;` You can replace the ternary op with `if(err != null) return err.color; return null;` if you have a preference that way. – Mad Physicist Jul 13 '16 at 14:57

Mad Physicist · Answer 3 · 2016-07-13T15:02:55.313

You have a couple of straightforward options available to you. One is to use a Map<Integer, String> and another is to to a linear search.

Using a map is probably much more efficient, but will take up more space. This is probably not an issue for a small number of enums.

public enum KDErrors {

  KDR_280(280, "Blue"),
  KDR_281(281, "Red"),
  KDR_282(282, "Green"),
  KDR_284(284, "Yellow"),

  private static Map<Integer, String> codeMap = new HashMap<>();

  private final int code;
  private final String color;

  private KDErrors(int code, String color) {
    this.code = code;
    this.color = color;
    codeMap.put(code, color);
  }

  public static String getColorByCode(int colorCode) {
   return codeMap(colorCode);
  }
}

Doing a linear search prevents you from having to allocate additional structures, but it will be a little slower for the average case than a lookup in a HashMap:

public enum KDErrors {

  KDR_280(280, "Blue"),
  KDR_281(281, "Red"),
  KDR_282(282, "Green"),
  KDR_284(284, "Yellow"),

  private final int code;
  private final String color;

  private KDErrors(int code, String color) {
    this.code = code;
    this.color = color;
  }

  public static String getColorByCode(int colorCode) {
   for(KDErrors err : KDErrors.values()) {
       if(err.getCode() == colorCode)
           return error.getColor();
    }
    return null;
  }
}

@Jens. Because it is generally a better approach than doing a linear search over a whole rack of items. It won't matter much if there are only a couple of items in the enum, but as the number grows, the map approach will start to shine. — Mad Physicist, Jul 13 '16 at 14:58
@Jens Thanks for the catch. It was just a stupid copy-n-paste error. Fixed now. — Mad Physicist, Jul 13 '16 at 15:03

Mapping inside an enum

3 Answers3