Procedural programming

In procedural programming, a unit could be an entire module, but it is more commonly an individual function or procedure.

Object-oriented programming

In object-oriented programming, a unit is often an entire interface, such as a class, or an individual method.^[5] By writing tests first for the smallest testable units, then the compound behaviors between those, one can build up comprehensive tests for complex applications.^[4]

Early detection of problems in the development cycle

Unit testing finds problems early in the development cycle. This includes both bugs in the programmer's implementation and flaws or missing parts of the specification for the unit. The process of writing a thorough set of tests forces the author to think through inputs, outputs, and error conditions, and thus more crisply define the unit's desired behavior.

Reduced cost

The cost of finding a bug before coding begins or when the code is first written is considerably lower than the cost of detecting, identifying, and correcting the bug later. Bugs in released code may also cause costly problems for the end-users of the software.^[8][9][10] Code can be impossible or difficult to unit test if poorly written, thus unit testing can force developers to structure functions and objects in better ways.

Test-driven development

In test-driven development (TDD), which is frequently used in both extreme programming and scrum, unit tests are created before the code itself is written. When the tests pass, that code is considered complete. The same unit tests are run against that function frequently as the larger code base is developed either as the code is changed or via an automated process with the build. If the unit tests fail, it is considered to be a bug either in the changed code or the tests themselves. The unit tests then allow the location of the fault or failure to be easily traced. Since the unit tests alert the development team of the problem before handing the code off to testers or clients, potential problems are caught early in the development process.

More frequent releases

Unit testing enables more frequent releases in software development. By testing individual components in isolation, developers can quickly identify and address issues, leading to faster iteration and release cycles.^[11]

Allows for code refactoring

Unit testing allows the programmer to refactor code or upgrade system libraries at a later date, and make sure the module still works correctly (e.g., in 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 identified quickly.

Detects changes which may break a design contract

Unit tests detect changes which may break a design contract.

Reduce uncertainty

Unit testing may reduce 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.

Documentation of system behavior

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's interface (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.

When software is developed using a test-driven approach, the combination of writing the unit test to specify the interface plus the refactoring activities performed after the test has passed, may take the place of formal design. Each unit test can be seen as a design element specifying classes, methods, and observable behavior.

Difficulty in setting up realistic and useful tests

Another challenge related to writing the unit tests is the difficulty of setting up realistic and useful tests. It is necessary to create relevant initial conditions so the part of the application being tested behaves like part of the complete system. If these initial conditions are not set correctly, the test will not be exercising the code in a realistic context, which diminishes the value and accuracy of unit test results.

Requires discipline throughout the development process

To obtain the intended benefits from unit testing, rigorous discipline is needed throughout the software development process.

Requires version control

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.

Requires regular reviews

It is also essential to implement a sustainable process for ensuring that test case failures are reviewed regularly and addressed immediately.^[13] 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.

Limitations for embedded system software

Unit testing embedded system software presents a unique challenge: Because the software is being developed on a different platform than the one it will eventually run on, you cannot readily run a test program in the actual deployment environment, as is possible with desktop programs.^[14]

Limitations for testing integration with external systems

Unit tests tend to be easiest when a method has input parameters and some output. It is not as easy to create unit tests when a major function of the method is to interact with something external to the application. For example, a method that will work with a database might require a mock up of database interactions to be created, which probably won't be as comprehensive as the real database interactions.^[15]

Java

Here is a set of test cases in Java that specify 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 over a number of test methods.

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class TestAdder {

    // can it add the positive numbers 1 and 1?
    @Test
    public void testSumPositiveNumbersOneAndOne() {
        Adder adder = new AdderImpl();
        assertEquals(2, adder.add(1, 1));
    }

    // can it add the positive numbers 1 and 2?
    @Test
    public void testSumPositiveNumbersOneAndTwo() {
        Adder adder = new AdderImpl();
        assertEquals(3, adder.add(1, 2));
    }

    // can it add the positive numbers 2 and 2?
    @Test
    public void testSumPositiveNumbersTwoAndTwo() {
        Adder adder = new AdderImpl();
        assertEquals(4, adder.add(2, 2));
    }

    // is zero neutral?
    @Test
    public void testSumZeroNeutral() {
        Adder adder = new AdderImpl();
        assertEquals(0, adder.add(0, 0));
    }

    // can it add the negative numbers -1 and -2?
    @Test
    public void testSumNegativeNumbers() {
        Adder adder = new AdderImpl();
        assertEquals(-3, adder.add(-1, -2));
    }

    // can it add a positive and a negative?
    @Test
    public void testSumPositiveAndNegative() {
        Adder adder = new AdderImpl();
        assertEquals(0, adder.add(-1, 1));
    }

    // how about larger numbers?
    @Test
    public void testSumLargeNumbers() {
        Adder adder = new AdderImpl();
        assertEquals(2222, adder.add(1234, 988));
    }
}

In this case the unit tests, having been written first, act as a design document specifying the form and behaviour of a desired solution, but not the implementation details, which are left 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 {
    public int add(int a, int b) {
        return a + b;
    }
}

Extreme programming

Unit testing is the cornerstone of extreme programming, 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. Extreme programming mandates a "test everything that can possibly break" strategy, over the traditional "test every execution path" method. This leads 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. Extreme programming simply recognizes that testing is rarely exhaustive (because it is often too expensive and time-consuming to be economically viable) and provides guidance on how to effectively focus limited resources.

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. Extreme programming'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.

Unit testing is also critical to the concept of Emergent Design. As emergent design is heavily dependent upon refactoring, unit tests are an integral component.

Unit testing frameworks

Unit testing frameworks are most often third-party products that are not distributed as part of the compiler suite. They help simplify the process of unit testing, having been developed for a wide variety of languages.

It is generally possible to perform unit testing without the support of a specific framework by writing client code that exercises the units under test and uses assertions, exception handling, or other control flow mechanisms to signal failure. Unit testing without a framework is valuable in that there is a barrier to entry for the adoption of unit testing; having scant unit tests is hardly better than having none at all, whereas once a framework is in place, adding unit tests becomes relatively easy.^[16] In some frameworks many advanced unit test features are missing or must be hand-coded.

Language-level unit testing support

Some programming languages directly support unit testing. Their grammar allows the direct declaration of unit tests without importing a library (whether third party or standard). Additionally, the boolean conditions of the unit tests can be expressed in the same syntax as boolean expressions used in non-unit test code, such as what is used for if and while statements.

Languages with built-in unit testing support include:

Languages with standard unit testing framework support include:

Some languages do not have built-in unit-testing support but have established unit testing libraries or frameworks. These languages include: