Add the Gradle build file.
Add .gitignore to the project.
Remove empty angle brackets.

This file does not compile with empty angle brackets. The list type should
be specified, or should not contain the angle brackets at all.
Move test code to src/ dir.

Grade, by default, expects to find test code in src/test dir.
Ignores a failing test. Should be fixed! ;)
Remove compiled files and library dependecies.

It is a good practice not to keep these files in the source code
repository. The compiled files can be generated locally any time, and
the dependencies, i.e. jar files, are going to be managed by Gradle,
which is going to download them if by the first time you need them.