Eh, I get why my phrasing might make it sound like they had a totally separate collections hierarchy; you're right, they don't. But I'd count the changes you list--especially the extension methods, which basically provide an entire Java 8-streams-like API that runs comfortably on top of even Android's superannuated Java--as a collections library in its own right. (Hell, even the fixes they do at the compiler level to maps makes the entire collections API feel a lot more like .NET, with its reified generics, and a lot less like Java's weird situation.)