Boundary testing exercises transitions between valid and invalid input. Values immediately below, at and above a boundary often expose incorrect comparisons.
Before you start
You should understand inputs, expected outputs and the boundaries of the component under test. State the risk a test should detect before choosing a tool or mock. Distinguish controlled dependencies from real integrations so the result does not imply more coverage than the test actually provides.
The practical goal is to reason through this situation: For a limit of ten items, test nine, ten and eleven. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Locate the transition
Write whether the limit is inclusive or exclusive.
Step 2: Test adjacent values
Use immediately below, exactly at and above the boundary.
Step 3: Control time if needed
Expiry logic must read the same clock the test controls.
Worked scenario
For a limit of ten items, test nine, ten and eleven.
If a token expires at time 100 and validity requires now less than 100, it is valid at 99 and invalid at 100 and 101. A test only at 50 cannot distinguish less-than from less-than-or-equal. Precision and units also matter when production timestamps use milliseconds or another resolution.
Common mistake
Testing only ordinary middle values misses inclusive-versus-exclusive mistakes.
Verify the behavior
Check both sides and equality under the documented units and comparison rule.
Interview exercise
Test a time expiry.
Answer and reasoning
Control the clock and check just before, exactly at and after the defined expiration rule.
Continue learning
Compare the scenario with the Software Testing interview questions and test your understanding with the Software Testing MCQs. For terminology and implementation details, consult the reference material.