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?

Closehauled

second one is a big anti-pattern and your first one does not initialize an EnumMap, but a Map (immutable one) – Annettaannette

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);

Deraign

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.

Annettaannette

any specific reason of doing it in static block? – Hawkinson

@Hawkinson how else? – Annettaannette

the first one also looks good. Is it better to do with static block – Hawkinson

Thanks got the answer. "Note that this Map.of method supports only a maximum of 10 key-value pairs." mention it in your article - baeldung.com/java-initialize-hashmap – Hawkinson

yeah agree...I was talking about Map.of which can store max of 10 mapping which is overcome with the ofEntries – Hawkinson

@Hawkinson A specific reason to use it could be to only have access to Java 8. Map.of() was introduced with Java 9. – Rockrose

hmm, static block or static method is the only way to do it in java 8 – Hawkinson

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;
}

Gorey