How does a type class help with composition? They do help with the expression problem (adding support for an "interface" after definition), and via parametric polymorphism they might give you a bit with regards to composing two traits.. but you do also have generics in Java, even if not as good as type classes.

So anyways, I don't see as big of a change here. But there was a Brian Goetz mail/presentation somewhere where he talked about adding "basically type classes" to Java? But unfortunately I couldn't find it for you now.