unit-test-boundary-conditions_skill

Overview

This skill provides patterns and ready-made approaches for unit testing boundary conditions and edge cases using pure JUnit 5. It focuses on numeric limits, null/empty strings, collection sizes, floating-point precision, date/time edges, array indexing, and basic concurrent scenarios. Use it to make tests explicit about limits and to prevent silent failures like overflow or NaN surprises.

How this skill works

The skill inspects common boundary categories (min/max values, zero, null, empty, whitespace, tiny/huge floats, first/last indices, out-of-bounds, leap dates, timezone transitions, and basic concurrency cases). It recommends parameterized tests, tolerance-based float assertions, and explicit checks just below, at, and just above boundaries. Examples show JUnit 5 annotations, AssertJ assertions, and techniques such as Math.addExact() to detect overflow and CopyOnWrite collections to exercise concurrency behavior.

When to use it

• Validating behavior at API or library boundaries
• Ensuring functions handle null, empty, and whitespace inputs
• Checking numeric overflow/underflow and precision issues
• Verifying collection handling for empty, single, and large sets
• Testing date/time edge cases like leap years and timezone transitions

Best practices

• List and document domain-specific boundaries before writing tests
• Use @ParameterizedTest to cover multiple boundary values succinctly
• Test values just below, at, and just above each boundary
• Use tolerance-based assertions for floating point comparisons
• Use Math.addExact()/subtractExact to reveal integer overflow
• Keep large-collection tests mindful of memory and runtime costs

Example use cases

• Unit tests for input validation functions covering null, empty, whitespace, and length limits
• Numeric algorithms that must behave correctly at Integer.MIN_VALUE/MAX_VALUE and near-zero thresholds
• Collection utilities that must return sensible values for empty, single, and large lists
• Date utilities that must handle leap days, min/max LocalDate and DST transitions
• Array and index-access code verifying first, last, and out-of-bounds behavior