Friday, January 04, 2008

Java Generics Broken? We Report, You Decide.

Since I've been taken to task in the past for my cogent observations of Java's, um, shall we say "unintuitive" behavior, particularly in the era of generics and autoboxing, I'm just going to put this one out there without making any (public (explicit)) value judgments.

So it seems that if you define a method thusly (note: requires Java 5 ("Gangly Geek") or better greater having a numerically higher version number):
public void doSomethingGenerically(Collection<Supertype>){...}
and you attempt to call it thusly:
List<SubtypeOfSupertype> bogus = new ArrayList<SubtypeOfSupertype>();
you get a compiler error saying you can't call that method with those arguments.

So my question here is: why the heck not?

Inheritance 101: a List isa Collection, right? A SubtypeOfSupertype isa Supertype, right? So why am I being told that an instance of type List<SubtypeOfSupertype> isn'ta Collection<Supertype>?

Paging Dr. Liskov...