domains / / JetBrains/kotlin

Test edge cases

Ensure tests verify both basic functionality and edge cases. Tests that only cover the happy path can miss critical bugs in boundary conditions and special scenarios.

raw .md Testing Kotlin

Ensure tests verify both basic functionality and edge cases. Tests that only cover the happy path can miss critical bugs in boundary conditions and special scenarios.

For each feature:

Identify edge cases (empty inputs, null values, boundary values)
Test all API functions and variations
Place assertions immediately after the code being tested

Example:

@Sample
fun contains() {
    // Basic functionality
    val string = "Kotlin 1.4.0"
    assertPrints(string.contains("K"), "true")
    assertPrints(string.contains("k"), "false")
    
    // Edge cases
    assertPrints("".contains(""), "true") // Empty receiver and parameter
    assertPrints("Kotlin".contains(""), "true") // Empty parameter
    assertPrints("".contains("Kotlin"), "false") // Empty receiver
    assertPrints("Kotlin".contains("Kotlin"), "true") // Identical strings
    assertPrints("Kotlin 2.0.0".contains("Kotlin"), "true") // Receiver contains parameter
    assertPrints("Kotlin".contains("Kotlin 2.0.0"), "false") // Parameter contains receiver
}

Include specific tests for error conditions using explicit assertions rather than try/catch blocks:

// Incorrect: Test passes even if cast doesn't fail
try {
    BNativeHeir as A.Companion
} catch (e: Exception) {
    assertTrue(e is ClassCastException)
}

// Correct: Test only passes if cast fails with expected exception
assertFailsWith<ClassCastException> {
    BNativeHeir as A.Companion
}

Comprehensive testing prevents code from “rotting” and ensures reliability across all usage scenarios.