Generic enum parameter in java. Is this possible?

Question

I am trying to write a generic function that will accept any enum, and put the values into a map for use in a drop down.

This is what I have so far, (for a specific enum), can my function enumToMap be rewritten generally to accept any enum type?

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {

    private enum testenum{
        RED,BLUE,GREEN
    }


    private static Map enumToMap(testenum e){
        HashMap result = new HashMap();
        List<testenum> orderReceiptKeys = Arrays.asList(e.values());
        List<String> orderReceiptValues = unCapsCase(orderReceiptKeys);
        for(int i=0;i<orderReceiptKeys.size();i++){
            result.put(orderReceiptKeys.get(i), orderReceiptValues.get(i));
        }
        return result;
    }

     /**
     * Converts a string in the form of 'TEST_CASE' to 'Test case'
     * @param s
     * @return
     */
    private static String unCapsCase(String s){
        StringBuilder builder = new StringBuilder();
        boolean first=true;
        for(char c:s.toCharArray()){
            if(!first)
                c=Character.toLowerCase(c);
            if(c=='_')
                c=' ';
            builder.append(c);
            first=false;
        }
        return builder.toString();
    }


    /**
     * Converts a list of strings in the form of 'TEST_CASE' to 'Test case'
     * @param l
     * @return
     */
    private static List<String> unCapsCase(List l){
        List<String> list = new ArrayList();

        for(Object o:l){
            list.add(unCapsCase(o.toString()));
        }

        return list;
    }


    public static void main(String[] args) throws Exception {

        try{
            testenum e=testenum.BLUE;
            Map map = enumToMap(e);
            for(Object k:map.keySet()){
                System.out.println(k.toString()+"=>"+map.get(k));
            }
        }catch(Exception e){
            e.printStackTrace();
        }


    }

}

Thanks for any suggestions.

Answer 1

Change the signature of your enumToMap method to something like this:

private static <E extends Enum<E>> Map<E, String> enumToMap(Class<E> enumType)

Then where you call e.values() use enumType.getEnumConstants() instead.

In your main method you can then call this method like:

Map<testenum, String> map = enumToMap(testenum.class);

As seanizer mentions, you should also be using EnumMap as the Map implementation rather than HashMap.

Answer 2

Don't use a HashMap with enums, this is what an EnumMap was designed for.

As written in the Map section of the Collection Trail in the Sun Java Tutorial:

EnumMap, which is internally implemented as an array, is a high-performance Map implementation for use with enum keys. This implementation combines the richness and safety of the Map interface with a speed approaching that of an array. If you want to map an enum to a value, you should always use an EnumMap in preference to an array.