Last week we kicked of a "Testing Skill Group" at LINEAS, a group for exchanging knowledge about testing. One question that came up over and over again in various flavors was: What tools are there for testing and analyzing your code? So here is my personal answer for this, in the approximately order I tend to introduce them into projects:
JUnit: JUnit is pretty much the basis for everything else. Its THE unit testing framework for Java, with great integration in IDEs, build tools and CI Server. I don't take the term 'unit' to seriously though. I use JUnit to execute all other kinds of tests as well.
Mockito: There are many Mocking Frameworks out there but I prefer this one. It has nice DSLish API and I find it nice to use. Only drawback is that in some special cases the standard API doesn't work and one has to use an alternative syntax.
PowerMock: I actually try not to introduce this into projects. You need PowerMock if you want to mock constructors, static or final methods. If you need this, PowerMock gets the job done, but its better not to need it.
CheckStyle: A static code analysis that finds lots of bad practices and can check lots of coding conventions too. Integrates in IDEs and CI server. There are other tools in this area which are worth considering: FindBugs and PMD. You can also use Checkstyle in order to gather simple metrics about your code.
JDepend: does static code analysis of the dependencies of your code. I use it to write tests against cycles between packages in the code and also in order to limit the dependencies to those I'm willing to accept in the code base. Some time ago I found out there are some limitations in JDepend resulting in dependencies that JDepend misses (I think it doesn't consider classes in annotations or something). Therefore I'm looking at DependencyFinder, which seems to be way more powerful, but is certainly harder to use. I actually build a little tool for visualizing dependencies based on DependencyFinder.
Cobertura/EclEmma: Cobertura and Emma are code coverage tools. I use them both. Emma in the form of EclEmma as an eclipse plugin and Cobertura in Jenkins, because we couldn't get Emma to work as we wanted on our Jenkins instance.
Sonar: It collects tons of metrics from your code and makes them available as a website. It actually to much numbers for my taste. In a serious project you can spend the whole day looking at numbers. What is really great about sonar is, that it tracks those numbers over time, so you can see if your average method length goes up or down over time. In some projects I configured a graph with the most important numbers plotted over time and added it to the main screen of Jenkins.