Not necessarily. Java currently ships two date libraries, for example: java.util.Date and java.time.*. There are some duplicating classes like Vector from the old times and ArrayList from new times. There are two I/O frameworks: old I/O (File, FileInputStream, etc) and nio (Path, FileSystem, etc). While I don't like the particular way Java did that, basically you just have to ship all old versions and keep them working and ship new versions with some adapters to ease migration.