java tool that calculates the percentage of code accessed by tests

 Tool that measures test coverage by instrumenting a Java code base and watching
 which lines of code are and are not executed as the test suite runs. In
 addition to identifying untested code and locating bugs, Cobertura can
 optimize code by flagging dead, unreachable code and can provide insights
 into how an API operates in practice. It is based on jcoverage.