ByteBuffer cannot be extended because constructors have default visibility

[Luke Hutchison]

The ByteBuffer class cannot be extended, because its two constructors both
have default visibility. This is problematic, since there are numerous
situations where it would be useful to be able to extend ByteBuffer.

For example, I needed to create a ByteBuffer that was backed by a
RandomAccessFile (without actually mapping the file to a MappedByteBuffer),
so that I could use a RandomAccessFile with a ByteBuffer API. As far as I
can tell, there is no way to do this. This led to a huge amount of
unnecessary work and duplicated code, since methods that accepted
ByteBuffer now needed to have a duplicated alternative that accepted
RandomAccessFile.

I just came across another example of this -- Google's Jimfs (in-memory
filesystem) has run into the same problem. Currently if you call
FileChannel.open(path) on a Jimfs path, then call map() on the file channel
to mmap it, Jimfs throws an UnsupportedOperationException. This is due to
ByteBuffer not being extendable: https://github.com/google/jimfs/issues/54

This seems like an unnecessary restriction, based on the source of
ByteBuffer. Can the ByteBuffer constructors please be made public?
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