Is there a corresponding immutable enumMap in guava?

Question

I recently learnt about the benefits of EnumMap in Java and would like to replace the existing ImmutableMap<OccupancyType, BigDecimal> to EnumMap. However, I'd also like the immutable property offered by ImmutableMap.

• Is there a variant, ImmutableEnumMap available in guava ?
• In terms of storage which one (EnumMap vs ImmutableMap) performs better ?
• I couldn't find a comparison of the two. I'd appreciate if someone can point me to a link or give some insights on the efficiency of the two data structures ?

Answer 1

Guava contributor here.

Guava doesn't currently have an ImmutableEnumMap variant, but if it did, it would probably just be a wrapper around an EnumMap. (That said, slightly better immutable implementations are possible.)

EnumMap will perform better than the basic ImmutableMap, in any event; it's difficult or impossible to beat.

(I'll file an issue to investigate adding an ImmutableMap variant for enum key types, though.)

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Update: Guava 14 adds Maps.immutableEnumMap().

Answer 2

I was just wanted to provide an example now that ImmutableEnumMap is in Guava 14.0, because it's a package-private class, so you can't do ImmutableEnumMap.of(). You have to do Maps.immutableEnumMap() instead.

private final ImmutableMap<MyEnum, String> myEnumMap = Maps.immutableEnumMap(ImmutableMap.of(
        MyEnum.A,   "A",
        MyEnum.B,   "B",
        MyEnum.C,   "C"
        ));

Not sure if there's a more natural syntax.

Answer 3

As the guava ImmutableEnumMap is still marked beta as of version 14, I would suggest using a unmodifiable view of a enum map and then throwing away the original reference to the enum map to ensure that it is immutable.

Example (in a constructor):

Map entries = new EnumMap <SomeEnum, T>(SomeEnum.class);
... // (fill in entries)
this.entries = Collections.unmodifiableMap(entries);