Its not about pieces of a single copyrighted work its about numerous pieces of separately copyrighted works that can be part of an overarching copyrighted "collective" an anthology of short stories is a good example. Each short story's copyright can remain the copyright of the individual authors, but the anthology as a whole is the copyright of whoever put the anthology together. In this case Java is the collective work, the APIs are the short stories.

To the point below assuming a mashup is made by licensing samples from other works (avoiding fair use issues for a second) the Mashup would be a copyrightable work. But the authors of the individual samples still retain their own copyright.