Without this requirement, generifying a class would require a “flag day” where all clients and subclasses have to be at least recompiled, if not modified – all

Without this requirement, generifying a class would require a “flag day” where all clients and subclasses have to be at least recompiled, if not modified – all at once. For a core class such as ArrayList, this essentially requires all the Java code in the world to be recompiled at once (or be permanently relegated to remain on Java 1.4.) Since such a “flag day” across the entire Java ecosystem was out of the question, we needed a generic type system that allowed core platform classes (as well as popular third-party libraries) to be generified without requiring clients be aware of their generification. (Worse, it wouldn’t have been one flag day, but many, since it is not the case that all the world’s code would have been generified in a single atomic transaction.) Another way to state this requirement is: it was not considered acceptable to orphan all the code out there that could have been generified, or make developers choose between generics and retaining the investment they’ve already made in existing code. By making generification a compatible operation, the investment in that code could be retained, rather than invalidated. It's great to see such care for the users, while at the same time improving the platform to make it one of the top performing ones.