Test-driven development (TDD) is a software development process that employs short cycles of writing a test, making it fail, then writing code to pass the test. This helps improve code design and reduces defects. Each new feature starts with writing a test case, then coding just enough to pass the test, and refactoring the code without changing external behavior. TDD produces test code documentation and allows reversible changes by starting over from the last stable state if new code causes errors.

1. Test Driven Development
Dr. Davut Çulha

2. Test-driven Development (TDD)
• Test-driven development (TDD) is a software
development process that employs very short
development cycle:
– Testing
– Coding
– Refactoring
• TDD = Test-first Programming + Refactoring.
– Test-first programming is used in XP.

3. Test-driven Development (TDD)
• TDD substantially reduces the incidence of
defects.
• It helps to improve the design of coding.
• It minimizes the usage of debuggers.

4. Write Test Code
• At each cycle of TDD, each new feature begins
with writing a test.
• If the feature is not very small, only a very
small part of it is taken as a feature. This rule
is applied for the rest of the initial feature.
• After writing the test, execute it. It will fail. If it
does not fail, repeat the process until failure.
• Test codes should not be dependent to each
other.

5. Write Some Code
• Before coding actual feature, you should have
taken the failure for the test.
• Write some code which is just enough to pass
the test. In other words, the failure will not be
taken.

6. Refactor
• Remove duplications.
• Improve your code quality.
• Use good naming.
• Increase readibility.
• Modularize the codes.
• Use design patterns.
• Do not change the external behaviour of the
code. Change only the internal behaviour of the
code to get technical excellence.

7. Reversible Changes
• If the new feature addition takes a complex
error, go to the last stable state.
• Implement the same feature from the
beginning.
• Instead of using debugging and solving the
problem, use reversible changes property of
TDD.
• Every change should be reversible. Start from
the last stable state.

8. Documentation
• Every test code is a kind of documentation.
• It describes the interface of the actual coding.
• Every test code is a sample program to
investigate the actual program.

9. Code Base
• In addition to actual codes, there are test
codes.
• The test codes should also be managed
carefully.
• The test codes should be kept as simple as you
can like the actual codes.