Provides edge case, corner case, boundary condition, and limit testing patterns for Java unit tests. Validates minimum/maximum values, null cases, empty collections, numeric overflow/underflow, floating-point precision, and off-by-one scenarios using JUnit 5 and AssertJ. Use when writing .java test

Unit Testing Boundary Conditions and Edge Cases Overview Systematic patterns for testing boundary conditions, corner cases, and limit values in Java using JUnit 5. Covers numeric boundaries, string edge cases, collection states, floating point precision, date/time limits, and off by one scenarios. When to Use Numeric min/max limits, null/empty/whitespace inputs Overflow/underflow validation, collection boundaries Off by one errors, floating point precision Instructions 1. Identify boundaries : List numeric limits (MIN VALUE, MAX VALUE, zero), string states (null, empty, whitespace), collection sizes (0, 1, many) 2. Apply parameterized tests : Use @ParameterizedTest with @ValueSource or @CsvSource for multiple boundary values 3. Test both sides of boundaries : Cover values just below, at, and just above each boundary 4. Run tests after adding each boundary category to catch issues early 5. Verify floating point precision : Use isCloseTo(expected, within(tolerance)) with AssertJ 6. Test collection states : Explicitly test empty (0), single (1), and many ( 1) element scenarios 7. Handle overflow/underflow : Use Math.addExact() and Math.subtractExact() to detect arithmetic overflow 8. Test date/time edges : Verify leap years, month boundaries, timezone transitions 9. Iterate based on failures : When a boundary test fails, analyze the error to discover additional untested boundaries; add test cases for the newly discovered edge conditions Examples Requires: junit jupiter , junit jupiter params , assertj core . Integer Boundary Testing String Boundary Testing Collection Boundary Testing Floating Point Boundary Testing Date/Time Boundary Testing Array Index Boundary Testing Best Practices Test at boundaries explicitly : don't rely on random testing Test null and empty separately from valid inputs Use parameterized tests for multiple boundary cases Test both sides of boundaries (just below, at, just above) Verify error messages for invalid boundary inputs Document why specific boundaries matter for your domain Test overflow/underflow for all numeric operations Constraints and Warnings Integer overflow : Use Math.addExact() to detect silent overflow Floating point precision : Never use exact equality; always use tolerance based assertions NaN behavior : NaN != NaN ; use Float.isNaN() or Double.isNaN() Collection size limits : Be mindful of memory with large test collections String encoding : Test with Unicode characters for internationalization Date/time boundaries : Account for timezone transitions and daylight saving Array indexing : Always test index 0, length 1, and out of bounds References [Integer.MIN VALUE/MAX VALUE](https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html) [Double.MIN VALUE/MAX VALUE](https://docs.oracle.com/javase/8/docs/api/java/lang/Double.html) [AssertJ Floating Point](https://assertj.github.io/assertj core features highlight.html assertions on numbers) [Boundary Value Analysis](https://en.wikipedia.org/wiki/Boundary value analysis) [references/concurrent testing.md](references/concurrent testing.md) Thread safety patterns [references/parameterized patterns.md](references/parameterized patterns.md) Off by one and parameterized examples

unit-test-boundary-conditions