Please add assertj-core to portage. AssertJ provides a rich and intuitive set of strongly-typed assertions to use for unit testing (either with JUnit or TestNG).
2.3.0 is now in the tree but this isn't the latest version. I avoided the latest for now because I wasn't sure whether it was compatible with the package I needed it for (testng) and it requires Java 8, which would have prevented stabilisation in the short term.
Solved with https://gitweb.gentoo.org/repo/gentoo.git/commit/dev-java/assertj-core/assertj-core-3.10.0.ebuild?id=1ccebaf11b4164add396ed2ffbbf78268ce7bb48