Prefer Kotlin for New Code

Severity: MEDIUM — Team standard from D92851254, D89896459 reviewer feedback

What to Look For

• New Java files being added (not modifications to existing)
• New test classes written in Java instead of Kotlin
• Kotlin code using !! (non-null assertion) operator
• Kotlin code using checkNotNull() instead of ?.let

When to Flag

• New Java file: "Why not Kotlin?" — new code should be Kotlin
• !! operator: Prefer safe calls (?.) or explicit null handling
• checkNotNull(x).doSomething(): Prefer x?.let { it.doSomething() }

Do NOT Flag

• Modifications to existing Java files (incremental migration OK)
• Java files that are part of OSS Fresco public API
• !! in test assertions where crash is intentional

Examples

// BAD — Non-null assertion can crash
val result = maybeNull!!.process()  // ❌ Crashes if null

// GOOD — Safe call with let
maybeNull?.let { result = it.process() }  // ✅ Safe

// GOOD — Elvis operator for default
val result = maybeNull?.process() ?: defaultValue  // ✅ Safe

Recommendation

1. New files: Always create in Kotlin
2. Null handling: Use ?.let, ?:, ?. instead of !! or checkNotNull()
3. Tests: Especially prioritize Kotlin for test code

Evidence

• D92851254: Reviewer asked "Why not Kotlin?" for new Java test class
• D89896459: PostprocessorProducer converted to idiomatic Kotlin