When you hear people talk about unit testing, they often talk about test coverage. Test coverage is the percentage of the application code base that is actually executed by the test cases.
In order to measure unit test code coverage, you need to execute the tests and keep track of the code that has or hasn't been executed.
Cobertura is a test coverage measurement utility for Java that does this. Other such utilities include jcoverage and Clover.
Cobertura works by instrumenting the Java bytecode, inserting code fragments of its own into already compiled code. These code fragments are executed while measuring code coverage during execution of test cases
Its usually assumed that a hundred percent test coverage is the ideal. This might not always be the case, and you should be aware of the cost/benefit trade-offs.
A simple counter-example is a simple getter method...