4833719: (bf) Views of MappedByteBuffers are not MappedByteBuffers, and cannot be forced

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-4833719

Provide covariant return type overrides for asReadOnlyBuffer(), compact(), duplicate(), and slice() [1]. If desirable, such overrides could also be provided for the bulk get() methods and the put*() methods.

Thanks,

Brian

[1] http://cr.openjdk.java.net/~bpb/4833719/webrev.00/