As others have mentioned, the Java test cases switch expected and actual output.

For Java, the starter method returns a Boolean instead of a boolean. Not a big deal, but unneccesary.

What is the advangage of using String.format over regular concatenation?

How do you get the fitness in the Java implementation? I've completed everything else.

Edit: https://docs.oracle.com/javase/8/docs/api/java/util/function/ToDoubleFunction.html

The "In other words, it can be any valid identifier character." portion of the instructions will vary based upon language.

This doesn't work for all cases, "⁀‿⁀" is technically a valid identifier.
http://stackoverflow.com/questions/11774099/legal-identifiers-in-java

The ternary is unnecessary here, "return Math.sqrt(n) == (int)Math.sqrt(n);" alone would do the exact same.