Object Literal Lambda

ID

kotlin.object_literal_lambda

Severity

low

Remediation Complexity

medium

Remediation Risk

low

Remediation Effort

medium

Resource

Code Smell

Language

Kotlin

Tags

best-practice, code-style

Description

Reports anonymous object : Interface { …​ } expressions that implement a single-abstract-method interface (Kotlin fun interface) with exactly one override. Such literals can be replaced by a lambda, which is the idiomatic form fun interface was designed to enable.

Rationale

fun interface Action { fun run() }
fun call(a: Action) = a.run()

// Verbose
call(object : Action {
    override fun run() = println("x")
})

fun interface was added so that callers could express the implementation as a lambda. Using an anonymous object negates that benefit, adds boilerplate (class name, override, function header, braces) and obscures the intent.

Remediation

fun interface Action { fun run() }

call { println("x") }

Replace the anonymous object with a lambda. If the implementation captures non-local state (e.g. has additional properties or multiple methods) it should remain an anonymous object — those usages are not flagged.

References