What is the best way to statically initialize an EnumMap in Java

Question

I need to statically initialize an EnumMap. I know two ways.

1. Using of() method of Map

   private static final Map<<EnumKey>, <Value>> TEST_MAP = Map.of(ENUM_CONST1, "Value1", ENUM_CONST2, "Value2");
   

2. Using double brace initialization

   private static final Map<<EnumKey>, <Value>> TEST_MAP = new EnumMap<>(EnumKey.class) {
       {
           put(ENUM_CONST1, "Value1");
           put(ENUM_CONST2, "Value2");
        }
   };
   

Is there any other cleaner and more efficient way?

Answer 1

A neat way to setup EnumMap is to define all the values in-line using Map.of or Map.ofEntries but note that this allocates a Map before the EnumMap constructor:

private static final EnumMap<YourEnum,String> A = new EnumMap<>(Map.of(
        YourEnum.ONE, "Value1",
        YourEnum.TWO, "Value2"
));

// Use Map.ofEntries for enums with more than 10 values:
private static final EnumMap<YourEnum,String> B = new EnumMap<>(Map.ofEntries(
        Map.entry(YourEnum.ONE, "Value1"),
        Map.entry(YourEnum.TWO, "Value2")
));

If wanting public access then wrap as unmodifiable Map (which is backed by EnumMap) or just pass back Map.of directly (but is not using an EnumMap).

public static final Map<YourEnum,String> C = Collections.unmodifiableMap(B);

Answer 2

do it in a static block:

private static final EnumMap<....> yourMap = ...

static {
   yourMap.put(....);
   yourMap.put(....)
}

There will be ways to do this rather differently (but they don't exist in the API yet), via Constant Dynamic.

Answer 3

Do it in a method:

private static final EnumMap<....> yourMap = yourMapMethod();

private static EnumMap<....> yourMapMethod() {
   EnumMap<....> yourMap = ...
   yourMap.put(....);
   yourMap.put(....);
   return yourMap;
}