Define Unit Test Case, Integration Test Case, System Test case

UNIT TEST CASES(UTC)
The unit test cases are very specific to a particular unit. The basic functionality of the unit is to be understood based on the requirements and the design documents. Generally, design document will provide a lot of information about the functionality of a unit. The design document has to be referred before a unit test case is written because it provides the actual functionality of how the system must behave, for given inputs.

INTEGRATION TEST CASES
Before designing the integration test cases the testers should go through the integration test plan. It will give complete idea of how to write integration test cases. The main aim of integration test cases is that it tests the multiple modules together. By executing these test cases the user can find out the errors in the interfaces between the modules.
The tester has to execute unit and integration test cases after coding.

SYSTEM TEST CASES
the system test cases meant to test the system as per the requirements; end to end. This is basically to make sure that the application works as per the software requirement specification. In system test cases, the testers are supposed to act as an end user. so, system test cases normally do concentrate on the functionality of the system, inputs are fed through the system and each and every check is performed using the system itself. Normally, the verifications are done by checking the database tables directly or running programs manually are not encouraged in the system test.
The system test must focus on functional groups, rather than identifying the program units. When it comes to system testing, it is assumed that the interfaces
between the modules are working fine.
Ideally the test cases are nothing but a union of the functionalities tested in the unit testing and the integration testing. Instead of testing the system inputs and outputs through database or external programs, everything is tested through the system itself. In system testing, the tester will mimic as an end user and hence checks the application through its output.
Sometimes, some of the integration and unit test cases are repeated in system testing also especially when the units are tested with test stubs before and not actually tested with other real modules, during system testing those cases will be performed again with real modules.

Validation phase - Unit Testing - UTC Document, UTC Checklist, Defect Recording

UTC Document
The Unit Test Case document consists of test case number, test case purpose, the procedure, the expected result and the actual result. Columns like Pass/Fail and Remarks are also present in UTC.

UTC Checklist
UTC checklist may be used while reviewing the UTC prepared by the programmer. As any other checklist, it contains a list of questions which can be answered in yes or no. The 'aspects' list can be referred to while preparing UTC checklist.
- Are test cases present for all form field validations?
- Are boundary conditions considered?
- Are error messages properly phrased?

Defect Recording
It can be done on the same document of UTC, in the column of 'Expected results'. this column can be duplicated for the next iterations of unit testing. Defect recording can also be done using some tools like Bugzilla in which defects are stored in the database. Defect recording needs to be done with care. It should be able to indicate the problem in clear, unambiguous manner and reproducing of the defects should be easily possible from the defect formation.

To conclude, exhaustive unit testing filters out the defects at an early stage in the development life cycle. It proves to be cost effective and improves quality of the software before the smaller pieces are put together to form an application as a whole. Unit testing should be done sincerely and meticulously.