Software Testing Functional Testing
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What is functional testing?

Functional testing verifies the behaviour of a system against its functional requirements. It asks: when the user does this, does the system do what it is supposed to do? The tester provides inputs, observes outputs and compares them with the expected results derived from the requirements, without needing to know how the code is written, which is why it is called black-box testing. Examples include checking that a login accepts valid credentials, that a discount is calculated correctly and that a report exports the right rows. Functional testing is the foundation of most test suites and covers what the software does, as opposed to how well it does it.

Functional versus non-functional testing

Levels at which functional testing happens

Black-box techniques

Because functional testers work from requirements, they use techniques to design a manageable set of tests. Equivalence partitioning groups inputs that should behave the same and tests one from each group. Boundary value analysis tests at the edges of ranges. Decision table testing covers combinations of rules, such as discount eligibility by customer type and order value. State transition testing checks that the system moves correctly through states such as order placed, paid, shipped and delivered. Use case testing follows complete user journeys. Error guessing and exploratory testing add the tester's experience and curiosity to find what scripted cases miss.

A worked example: a loan eligibility form

Suppose the requirement says an applicant is eligible when age is between 21 and 60 inclusive and monthly income is at least Rs. 25,000. Equivalence partitions for age are below 21, 21 to 60 and above 60. Boundary values are 20, 21, 60 and 61. Income has partitions below and at or above 25,000, with boundaries at 24,999 and 25,000. A decision table of the two conditions gives four combinations, and only the combination of valid age and sufficient income gives eligible. The tester designs cases for each boundary and combination, and also checks input validation, such as letters in the age field, blank fields and negative numbers.

Decision table for the loan example

A functional testing process

1. Study the requirements and clarify anything ambiguous.
2. Identify scenarios and design test cases with the techniques above.
3. Prepare test data and the test environment.
4. Execute the cases and compare actual with expected results.
5. Report defects with clear steps and evidence.
6. Retest fixes, run regression checks and report coverage and results.

What functional testing usually covers

• → Input validation and error messages.
• → Business rules and calculations.
• → Navigation and user flows.
• → Data saved, updated and displayed correctly.
• → Permissions and role-based access.
• → Integration with other modules and services.

Common mistakes

Types of functional testing

Checklist for a functional test of any form

Interview-style questions

Tips for beginners

• → Always trace each test back to a requirement or story.
• → Ask about unclear rules before writing cases.
• → Test what is not allowed as carefully as what is allowed.