Introduction

Unit testing is implemented against the smallest testable element (units) of the software, and involves testing the internal structure such as logic and data flow, and the unit's function and observable behaviors. Designing and implementing tests focused on a unit's internal structure rely upon the knowledge of the unit's implementation (white-box approach). The design and implementation of tests to verify the unit's observable behaviors and functions do not rely upon a knowledge of the implementation and therefore is known as black-box approach. 

Both approaches are used to design and implement the different types of tests (see Technique: Type of Tests) needed to successfully and completely test units. 

See also Guideline: Test Case for additional information on deriving test cases for unit test.

White-Box Test Approach

A white-box test approach should be taken to verify a unit's internal structure. Theoretically, you should test every possible path through the code, but that is possible only in very simple units. At the very least you should exercise every decision-to-decision path (DD-path) at least once because you are then executing all statements at least once. A decision is typically an if-statement, and a DD-path is a path between two decisions.

To get this level of test coverage, it is recommended that you choose test data so that every decision is evaluated in every possible way. 

Use code-coverage tools to identify the code not exercised by your white box testing. Reliability testing should be done simultaneously with your white-box testing.

See Guideline: Test Case for additional information 

Black-Box Test Approach

The purpose of a black-box test is to verify the unit's specified function and observable behavior without knowledge of how the unit implements the function and behavior. Black-box tests focus and rely upon the unit's input and output.

Deriving unit tests based upon the black-box approach utilizes the input and output arguments of the unit's operations, and / or output state for evaluation. For example, the operation may include an algorithm (requiring two values as input and return a third as output), or initiate change in an object's or component's state, such as adding or deleting a database record.  Both must be tested completely. To test an operation, you should derive sufficient test cases to verify the following:

• an appropriate value was returned by the operation for each valid value used as input
• an appropriate value was returned by the operation for each invalid value used as input
• an appropriate output state occurs for each valid input state
• an appropriate output state occurs for each invalid input state

Use code-coverage tools to identify the code not exercised by your white box testing. Reliability testing should be done simultaneously with your black-box testing.

See Guideline: Test Case for additional information