为什么EnumSet是作为抽象类实现的，而EnumMap是作为具体类实现的？

Question

我想知道，为什么EnumSet被实现为抽象类并且EnumMap被实现为具体类？

Answer 1

EnumSet actually has two implementations - one for enums with 64 or less elements (flags indicating presence of values in the set are stored as long ) and another one for other enums (flags are stored as long[] ). Factory methods of EnumSet return one of them depending on the enum class passed in.

Such an optimization doesn't make sense for EnumMap (since array to store values is needed anyways), therefore EnumMap is a concrete class.

Answer 2

EnumSet uses two implementations, RegularEnumSet and JumboEnumSet , depending on the size of your enum (64 elements being the threshold). The factory methods delegate to the appropriate implementation.

EnumMap on the other hand works the same for all enum types (it keeps an array of all enum items and an equally-sized array of values), so there's no need for an abstract class.