PIT (a.k.a. pitest) is a mutation testing tool which is fairly easy to set up (in maven or gradle) for most java applications.
The basic idea with mutation testing is that the tool will iterate over each line of code, changing that 1 line, and then run all the tests against the mutated code base. If the changed line provokes an error in at least one test, it’s considered covered. If no test fails, then the line is not covered.
It’s possible to configure PIT to ignore your slow integration tests or end-to-end tests, which is a good idea, because PIT will run the same tests over and over again. With isolated unit tests, it’s pretty fast and a great coverage tool.
I tried it out on my interface-it project. Here’s a link to my pom.xml file which uses the pitest maven plugin. I struggled a little with the configuration of target classes, and their getting started document was slightly off, but it did not take long to set up.
Here are a few screen captures of the resulting report, which includes drill-down to see each line of code and an explanation of the mutation for the line and the result of the mutation :