Java: Generic method for Enums

Question

Help me understand generics. Say I have two enums as inner classes like so:

public class FoodConstants {
    public static enum Vegetable {
        POTATO,BROCCOLI,SQUASH,CARROT;
    }

    public static enum Fruit {
        APPLE,MANGO,BANANA,GUAVA;
    }
}

Instead of having both enums implement an interface, and have to implement the same method twice, I would like to have a method in the outer class that does something like:

public <e> String getEnumString<Enum<?> e, String s) {
    for(Enum en: e.values()) {
        if(en.name().equalsIgnoreCase(s)) {
            return s;
        }
    }
    return null;
}

However this method does not compile. What I am trying to do is find out if a string value is the name of an enumerated value, in ANY enum, whether it's Vegetable, Fruit, what not. Regardless of whether this is in fact a redundant method, what is wrong with the one I am trying to (re)write?

Basically I would like to do this:

public class FoodConstants {
    public static enum Vegetable {
        POTATO,BROCCOLI,SQUASH,CARROT;
    }

    public static enum Fruit {
        APPLE,MANGO,BANANA,GUAVA;
    }

    public <e> String getEnumString<Enum<?> e, String s) {
        for(Enum en: e.values()) {
            if(en.name().equalsIgnoreCase(s)) {
                return s;
            }
        }
        return null;
    }
} //end of code

Answer 1

public static <E extends Enum<E>>
String getEnumString(Class<E> clazz, String s){
  for(E en : EnumSet.allOf(clazz)){
    if(en.name().equalsIgnoreCase(s)){
      return en.name();
    }
  }
  return null;
}

The original has a few problems:

1. It accepts an instance of the enum instead of the class representing the enum which your question suggests you want to use.
2. The type parameter isn't used.
3. It returns the input instead of the instance name. Maybe returning the instance would be more useful -- a case-insensitive version of Enum.valueOf(String).
4. It calls a static method on an instance so you can iterate. EnumSet does all the reflective stuff for you.