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In computer programming, unit testing is a method of testing that verifies the individual units of source code are working properly. A unit is the smallest testable part of an application. In procedural programming a unit may be an individual program, function, procedure, etc., while in object-oriented programming, the smallest unit is a method, which may belong to a base/super class, abstract class or derived/child class.

Ideally, each test case is independent from the others; Double objects like stubs, mock or fake objects as well as test harnesses can be used to assist testing a module in isolation. Unit testing is typically done by software developers to ensure that the code they have written meets software requirements and behaves as the developer intended.

Unit testing is a software development process in which the smallest testable parts of an application, called units, are individually and independently scrutinized for proper operation. Unit testing is often automated but it can also be done manually. This testing mode is a component of Extreme Programming (XP), a pragmatic method of software development that takes a meticulous approach to
building a product by means of continual testing and revision.

Unit testing involves only those characteristics that are vital to the performance of the unit under test. This encourages developers to modify the source code without immediate concerns about how such changes might affect the functioning of other units or the program as a whole. Once all of the units in a program have been found to be working in the most efficient and error-free manner possible, larger
components of the program can be evaluated by means of integration testing.

Unit testing can be time-consuming and tedious. It demands patience and thoroughness on the part of the development team. Rigorous documentation must be maintained. Unit testing must be done with an awareness that it may not be possible to test a unit for every input scenario that will occur when the program is run in a real-world environment.

Benefits

The goal of unit testing is to isolate each part of the program and show that the individual parts are correct. A unit test provides a strict, written contract that the piece of code must satisfy. As a result, it affords several benefits. Unit tests find problems early in the development cycle.

Facilitates change

Unit testing allows the programmer to refactor code at a later date, and make sure the module still works correctly (i.e. regression testing). The procedure is to write test cases for all functions and methods so that whenever a change causes a fault, it can be quickly identified and fixed.

Readily-available unit tests make it easy for the programmer to check whether a piece of code is still working properly. Good unit test design produces test cases that cover all paths through the unit with attention paid to loop conditions.

In continuous unit testing environments, through the inherent practice of sustained maintenance, unit tests will continue to accurately reflect the intended use of the executable and code in the face of any change. Depending upon established development practices and unit test coverage, up-to-the-second
accuracy can be maintained.

Simplifies integration

Unit testing helps to eliminate uncertainty in the units 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, integration testing becomes much easier.

A heavily debated matter exists in assessing the need to perform manual integration testing. While an elaborate hierarchy of unit tests may seem to have achieved integration testing, this presents a false sense of confidence since integration testing evaluates many other objectives that can only be proven through the human factor. Some argue that given a sufficient variety of test automation systems,
integration testing by a human test group is unnecessary. Realistically, the actual need will ultimately depend upon the characteristics of the product being developed and its intended uses. Additionally, the human or manual testing will greatly depend on the availability of resources in the organization.

Documentation

Unit testing provides a sort of living documentation of the system. Developers looking to learn what functionality is provided by a unit and how to use it can look at the unit tests to gain a basic understanding of the unit API.

Unit test cases embody characteristics that are critical to the success of the unit. These characteristics can indicate appropriate/inappropriate use of a unit as well as negative behaviors that are to be trapped by the unit. A unit test case, in and of itself, documents these critical characteristics, although many software development environments do not rely solely upon code to document the product in development.

On the other hand, ordinary narrative documentation is more susceptible to drifting from the implementation of the program and will thus become outdated (e.g. design changes, feature creep, relaxed practices to keep documents up to date).

Design

When software is developed using a test-driven approach, the Unit-Test may take the place of formal design. Each unit test can be seen as a design element specifying classes, methods, and observable behaviour. The following Java example will help illustrate this point.

Here is a test class that specifies a number of elements of the implementation. First, that there must be an interface called Adder, and an implementing class with a zero-argument constructor called AdderImpl. It goes on to assert that the Adder interface should have a method called add, with two integer parameters,
which returns another integer. It also specifies the behaviour of this method for a small range of values.

 public class TestAdder {
   public void testSum() {
     Adder adder = new AdderImpl();
     assertTrue(adder.add(1,1) == 2);
     assertTrue(adder.add(1,2) == 3);
     assertTrue(adder.add(2,2) == 4);
     assertTrue(adder.add(0,0) == 0);
     assertTrue(adder.add(-1,-2) == -3);
     assertTrue(adder.add(-1,1) == 0);
     assertTrue(adder.add(1234,988) == 2222);
   }
 }

In this case the unit test, having been written first, acts as a design document specifying the form and behaviour of a desired solution, but not the implementation details, which are left as an exercise for the programmer. Following the 'do the simplest thing that could possibly work' practice, the easiest solution that will make the test pass is shown below.

 interface Adder {
   int add(int a, int b);
 }
 class AdderImpl implements Adder {
   int add(int a, int b) {
      return a + b;
   }
 }

Unlike other diagram-based design methods, using a unit-test as a design has one significant advantage.
The design document (the unit-test itself) can be used to verify that the implementation adheres to the design.
UML suffers from the fact that although a diagram may name a class Customer, the developer can call the class Wibble and nothing in the system would note this discrepancy. With the unit-test design method, the tests will never pass if the developer does not implement the solution according to the design.

It is true that unit-testing lacks some of the accessibility of a diagram, but UML diagrams are now easily generated for most modern languages by free tools (usually available as extensions to IDEs). Free tools, like those based on the xUnit framework, outsource to another system the graphical rendering of a view for human
consumption.

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 often writes code that interacts with the database. This is a mistake, because a unit test
should never go outside of its own class boundary. Instead, the software developer should create an abstract interface around the database connection, and then implement that interface with their own mock object. By abstracting this necessary attachment from the code (temporarily reducing the net effective coupling), the
independent unit can be more thoroughly tested than may have been previously achieved. This results in a higher quality unit that is also more maintainable.

Limitations of unit testing

Testing cannot be expected to catch every error in the program - it is impossible to evaluate all execution paths for all but the most trivial programs. The same is true for unit testing. Additionally, by definition unit testing only tests the functionality of the units themselves. Therefore it will not catch integration errors, or broader
system level errors (such as functions performed across multiple units, or non-functional test areas such as performance).
 Unit testing is more effective if it is used in conjunction with other software testing activities. Like all forms of software testing, unit tests can only show the presence of errors; it cannot show the absence of errors.
Software testing is a combinatorial problem. For example, every boolean decision statement requires at least two tests:
one with an outcome of "true" and one with an outcome of "false". As a result, for every line of code written, programmers often need 3 to 5 lines of test code.

To obtain the intended benefits from unit testing, a rigorous sense of discipline is needed throughout the software development process. It is essential to keep careful records not only of the tests that have been performed, but also of all changes that have been made to the source code of this or any other unit in the software. Use of a version control system is essential. If a later version of the unit fails a particular test that it had previously passed, the version-control software can provide a list of the source code changes (if any) that have been applied to the unit since that time.

It is also essential to implement a sustainable process for ensuring that test case failures are reviewed daily and addressed immediately. If such a process is not implemented and ingrained into the team's workflow, the application
will evolve out of sync with the unit test suite ― increasing false positives and reducing the effectiveness of the test suite.

Applications

Extreme Programming

Unit testing is the cornerstone of Extreme Programming (XP), which relies on an automated unit testing framework.
This automated unit testing framework can be either third party, e.g. xUnit, or created within the development group.

Extreme Programming uses the creation of unit tests for test-driven development. The developer writes a unit test that exposes either a software requirement or a defect. This test will fail because either the requirement isn't implemented yet, or because it intentionally exposes a defect in the existing code. Then, the developer writes the simplest code to make the test, along with other tests, pass.

Most code in a system is unit tested, but not necessarily all paths through the code. XP mandates a 'test everything that can possibly break' strategy, over the tradition 'test every execution path' method. This leads XP developers to develop fewer tests than classical methods, but this isn't really a problem, more a restatement of fact, as classical methods have rarely ever been followed methodically enough for all execution paths to have been thoroughly tested.
XP simply recognizes that testing is rarely exhaustive (because often that is too expensive and time consuming to be economically viable), and provides guidance on how to effectively focus the limited resources we can afford expend on
the problem.

Crucially, the test code is considered a first class project artifact in that it is maintained at the same quality as the implementation code, with all duplication removed. Developers release unit testing code to the code repository in conjunction
with the code it tests. XP's thorough unit testing allows the benefits mentioned above, such as simpler and more confident code development and refactoring, simplified code integration, accurate documentation, and more modular designs.
These unit tests are also constantly run as a form of regression test.

Six Rules of Unit Testing

   1. Write the test first
   2. Never write a test that succeeds the first time
   3. Start with the null case, or something that doesn't work
   4. Don't be afraid of doing something trivial to make the test work
   5. Loose coupling and testability go hand in hand
   6. Use mock objects