Cast Nullable To NonNullable

ID

kotlin.cast_nullable_nonnull

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Kotlin

Tags

correctness, null-safety

Description

Reports casts of a nullable-typed expression to a non-null type using the unsafe as operator, which throws NullPointerException if the value is null at runtime, bypassing Kotlin’s null-safety system.

Rationale

fun process(x: String?) {
    // Bad — throws NPE if x is null; defeats Kotlin null safety
    val s = x as String  // FLAW

    // Good — as? integrates with the null-safety type system
    val s = x as? String ?: return
}

Remediation

Replace x as Type with x as? Type and handle the potential null via the Elvis operator (?:) or a null check. If a null source should be a programming error, use x!! with a descriptive error message rather than a silent cast.

References