Convert null to Enum.NULL

Question

I have an Enum like this:

public enum Type{
    CSV, EXCEL, PDF, URL, NULL
}

now I read a String from an xml-file and use Type.valueOf(string); to parse the value to the enum. if this part does not exist in the xml-file, the string is null and not "null". Is there a way to convert null to "null" or should change the return null; to return "null";? or should null not even be a part of the enum?

Answer 1

What you should do is use something else than NULL and parse that case differently:

public enum Type {
  CSV, EXCEL, PDF, URL, NONE;
  public static Type from(String text) {
    if (text == null) {
      return NONE;
    } else {
      return valueOf(text.toUpperCase());
    }
  }
}

Or better yet, use optionals:

public enum Type {
  CSV, EXCEL, PDF, URL; // Note the absence of NULL/NONE/WHATEVER
  public static Optional<Type> from(String text) {
    return Optional.ofNullable(text)
      .map(String::toUpperCase)
      .map(Type::valueOf);
  }
}

Answer 2

If you call Type.valueOf(null) you'll get a NullPointerException. So I would suggest two solutions:

1. write a utility function which includes a nullcheck:

   public static Type getType(String name) {
       if (name == null)
           return Type.NULL;
       else
           return Type.valueOf(name.toUpperCase());
   }
   

2. create a map from the name to the field (except for NULL) and add a getter to your enum:

   public enum Type {
       CSV, EXCEL, PDF, URL, NULL;
   
       private static final Map<String, Type> TYPE_BY_NAME = new HashMap<>();
       static {
           TYPE_BY_NAME.put(CSV.name(), CSV);
           TYPE_BY_NAME.put(EXCEL.name(), EXCEL);
           TYPE_BY_NAME.put(PDF.name(), PDF);
           TYPE_BY_NAME.put(URL.name(), URL);
           TYPE_BY_NAME.put(null, NULL);
       }
   
       public static Type getType(String name) {
           String s = name == null ? null : name.toUpperCase();
           return TYPE_BY_NAME.get(s);
       }
   

The second solution needs a bit more typing but you don't get an exception if the name does not exist. If you're sure that only the fields of your enum are given I would prefer the first solution.