ArrayList etc do have add(), and they implement it by re-allocating the backing array once capacity is exceeded.

In practice, you'd use the ArrayList anyway. I don't think it's worthwhile comparing Go and Java "language-only", because the standard library is as much a part of the language definition as the fundamental syntax; indeed, what goes where is largely arbitrary. E.g. maps in Go are fundamental, but the primary reason is that they couldn't be implemented as a library in Go with proper type safety, due to the lack of generics.