Why should I care that Java doesn't have reified generics?

Question

This came up as a question I asked in an interview recently as something the candidate wished to see added to the Java language. It\'s commonly-identified as a pain that Jav

Answer 1

Type safety comes to mind. Downcasting to a parametrized type will always be unsafe without reified generics:

List myFriends = new ArrayList();
myFriends.add("Alice");
getSession().put("friends", myFriends);
// later, elsewhere
List myFriends = (List) getSession().get("friends");
myFriends.add(new Friend("Bob")); // works like a charm!
// and so...
List myFriends = (List) getSession().get("friends");
for (String friend : myFriends) print(friend); // ClassCastException, wtf!? 

Also, abstractions would leak less - at least the ones which may be interested in runtime information about their type parameters. Today, if you need any kind of runtime information about the type of one of the generic parameters you have to pass its Class along as well. That way, your external interface depends on your implementation (whether you use RTTI about your parameters or not).