Software Unit Testing is about testing of individual units of source code. Those units are generally methods and they are grouped by class names (called as Test Case).
For example, if you have an XYZUtility class which having methodA, methodB, methodC methods, you will potentially have an XYZUtilityTestCase class having methodATest, methodBTest and methodCTest tests.
Unit tests must cover as much code as they can while running, but it is not the primary purpose. Code coverage is prefferred to be high as a result but the main purpose of the unit tests are:
- Testing output of a function with specific inputs. Those inputs must include boundary (min/max) values and potential exceptional values (divide by 0 etc.). And the inputs are also preffered to be chosen for covering all condition cases of the method.
- Documenting the software. One of the self-documenting code development techniques is unit testing. It shows usage of the code section.
- Increasing modularity, flexibility and extensibility of the code along with concerning the developer with the interface before implementation, in Test Driven Development (TDD) approach.
There are also 5 important properties for unit tests that must be obeyed according to Andy Hunt's Pragmatic Unit Testing book.
Unit test results (pass/fail) must be controlled automatically with tools. And generally more than one tests are need to be run together. A test should not have a manual step which stops execution of all tests.
A unit test must be repeatable. In other words, if it passes on a certain version of code, it must always pass with that code or if it fails on a certain version of code, it must always fail with that code. Database changes or other external services or files etc. should not change the result of the unit test.
Unit test results must be independent of other unit tests. If a unit test result changes after changing the running order or other environment change (database data etc.), it can not be called as a unit test.
A unit test must control every condition as it can (all possibilities, exceptions, boundaries ...). Its code coverage must be as much as it can and controlled conditions are preferred to be logical and functional.
Unit test code must obey coding standards of the project which it belongs to, because it also is a professional code. Furthermore, it was stated before that unit test is another way of self-documenting code. Clean and understandable unit test code will bring better documentation.
As a last word, for providing repeatable and independent unit tests, using mock objects is a good practice. Mock objects replaces the external sources (database, services, etc.) and returns predefined results. A good object oriented design having required amount of interfaces will simplfy the creation of mock objects. You can also use some API's like EasyMock or Mockito.