There are of course language features in Scala that are not available in Java.
You cannot do the things that some of these features provide directly in Java.
If the effort of emulating this in java is too big compared to the effort in Scala I would for the sake of this question like to say that you 'cannot do it' in Java.
Otherwise you can do anything in any language which I don't think is what you are asking.
The main thing you cannot do in Java is write as little code in Java as in Scala to accomplish the same thing. That's a big thing for me.
On the other hand you can always write the same amount of code in Scala as in Java to accomplish the same thing, i.e. it never gets worse.
It's a huge thing that Java does not have first class functions.
For instance the Strategy pattern in Java (interface, implementation, delegate to interface) is not necessary when you have functions. You just pass a function to another function. (the function is the strategy)
You can sort of do the same thing with anonymous classes in Java as functions but it's so cumbersome that you really don't want to and its only on the surface that that is the same thing, you cannot just do partial application, currying, composition and so many other things you would like to do with functions once you get used to them.
type inference, expressions, vals, case classes, first class functions, pattern matching, traits, tuples, typeclasses, type constructors, implicits, objects and so many other features are all boilerplate cutters.
For instance in java you can't mixin functionality like you can with Scala traits. You will have to use composition/delegation. I wrote a post for a simple little pattern in Akka here
http://letitcrash.com/post/30585282971/discovering-message-flows-in-actor-systems-with-the
and someone asked recently how to write this in Java. It's a lot more work. I wouldn't even want to try it. Sure it's possible but I just would not have the strength for it ;-)
And then there are features like macros that are not possible in Java. The new Typed Channels feature in Akka shows some of the possibilities of macros:
http://doc.akka.io/docs/akka/snapshot/scala/typed-channels.html
You can't simply write something in Java that will be checked compile time like that.
I say try it out ;-)