May Be Const

ID

kotlin.may_be_const

Severity

low

Remediation Complexity

medium

Remediation Risk

low

Remediation Effort

medium

Resource

Code Smell

Language

Kotlin

Tags

best-practice, efficiency

Description

Reports val properties at file or object scope whose initialiser is a compile-time literal (numeric, character, boolean, or a non-interpolated string) and which therefore qualify to be declared const val.

Rationale

val PI = 3.14
val NAME = "kotlin"

object Constants {
    val LIMIT = 10
}

A plain val with a literal initialiser still allocates a field, executes its initialiser at first access, and cannot be referenced from places that require a compile-time constant. The const modifier removes all of those costs and unlocks several legitimate uses that the plain val cannot support.

In particular, const val:

  • is inlined at the use site by the Kotlin compiler, so the field is gone from the generated bytecode entirely;

  • can be referenced from annotation arguments, which would otherwise require a string literal;

  • can be referenced from when branches and other contexts that demand a constant expression;

  • takes part in cross-module constant folding, so consumer modules see the literal value directly.

Remediation

const val PI = 3.14
const val NAME = "kotlin"

object Constants {
    const val LIMIT = 10
}

Add the const modifier to any qualifying property. The candidates are val declarations at file scope, inside a named object, or inside a companion object whose initialiser is a compile-time literal and whose type is a primitive or String. Instance properties of a class cannot be const and are not reported.

References