Unit Testing
In computer programming, a unit test is a method of testing the correctness of a particular module of source code.
The idea is to write test cases for every non-trivial function or method in the module so that each test case is separate from the others if possible. This type of testing is mostly done by the developers.
Benefits
The goal of unit testing is to isolate each part of the program and show that the individual parts are correct. It provides a written contract that the piece must satisfy. This isolated testing provides four main benefits:
Encourages change
Unit testing allows the programmer to refactor code at a later date, and make sure the module still works correctly (regression testing). This provides the benefit of encouraging programmers to make changes to the code since it is easy for the programmer to check if the piece is still working properly.
Simplifies Integration
Unit testing helps eliminate uncertainty in the pieces themselves and can be used in a bottom-up testing style approach. By testing the parts of a program first and then testing the sum of its parts will make integration testing easier.
Documents the code
Unit testing provides a sort of "living document" for the class being tested. Clients looking to learn how to use the class can look at the unit tests to determine how to use the class to fit their needs.
Separation of Interface from Implementation
Because some classes may have references to other classes, testing a class can frequently spill over into testing another class. A common example of this is classes that depend on a database; in order to test the class, the tester finds herself writing code that interacts with the database. This is a mistake, because a unit test should never go outside of its own class boundary. As a result, the software developer abstracts an interface around the database connection, and then implements that interface with their own Mock Object. This results in loosely coupled code, thus minimizing dependencies in the system.
Limitations
It is important to realize that unit-testing will not catch every error in the program. By definition, it only tests the functionality of the units themselves. Therefore, it will not catch integration errors, performance problems and any other system-wide issues. In addition, it may not be trivial to anticipate all special cases of input the program unit under study may receive in reality. Unit testing is only effective if it is used in conjunction with other software testing activities.
Integration Testing
It is the phase of software testing in which individual software modules are combined and tested as a group. It follows unit testing and precedes system testing. takes as its input modules that have been checked out by unit testing, groups them in larger aggregates, applies tests defined in an Integration test plan to those aggregates, and delivers as its output the integrated system ready for system testing.
Purpose
The purpose of Integration testing is to verify functional, performance and reliability requirements placed on major design items. These "design items", i.e. assemblages (or groups of units), are exercised through their interfaces using Black box testing, success and error cases being simulated via appropriate parameter and data inputs. Simulated usage of shared data areas and inter-process communication is tested, individual subsystems are exercised through their input interface. All test cases are constructed to test that all components within assemblages interact correctly, for example, across procedure calls or process activations.
The overall idea, is the "building block" approach in which verified assemblages are added to a verified base which is then used to support the Integration testing of further assemblages.
