Why EnumMap constructor needs class argument?

Question

EnumMap class constructor needs class as the argument. Most of the times K.class passed as the argument. I am still not getting what is the reason for accepting this as argument instead of deducing from K.

Thanks
-- pkc

Answer 1

The Map thus knows all possible keys. It's called (internally) the keyUniverse. The comments says:

All of the values comprising K. (Cached for performance)

Answer 2

The implementations of EnumMap needs metainformation about the enum, in particular the number of values. The Class object provides this information (IMO it would have been better to go for a specific enum descriptor type). If you don't have the Class available, you can always use HashMap at some penalty. I guess you could create a growable/uncommitted EnumMap-like Map.

Answer 3

Tom's answer is correct, but to address your other point: the reason this information can't just be deduced from the type parameter, K, is due to type erasure.

Answer 4

Generics is a compile time feature, however this K class is needed at runtime, something generics won't do in this case.

Answer 5

As others point out generics are a compiler feature. The jvm has no real support for generics itself. This means that the generic information cannot be used at runtime.

For the EnumMap this means that you get a EnumMap at runtime without any information about the K. This limitation of java generics can be worked around by passing the classes of the Generic arguments to a constructor as the class objects still exist at runtime.