--- title: Test & Set | daily.dev description: A developer shares a reusable Kotlin extension function called testAndSet() that wraps the common compareAndSet() retry loop pattern used with AtomicReference.... canonical: https://daily.dev/posts/test-set-ahjohbn3v twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Test & Set | daily.dev og:description: A developer shares a reusable Kotlin extension function called testAndSet() that wraps the common compareAndSet() retry loop pattern used with AtomicReference.... og:url: https://daily.dev/posts/test-set-ahjohbn3v og:image: https://api.daily.dev/og/posts/ahjohbn3V.png og:image:alt: Test & Set og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Test & Set **[Square](https://daily.dev/sources/publicobject)** · 3 min read · 0 upvotes · 0 comments ## Summary A developer shares a reusable Kotlin extension function called testAndSet() that wraps the common compareAndSet() retry loop pattern used with AtomicReference. The function takes a condition and new value, uses tailrec instead of a manual while loop, and returns the previous value rather than a boolean, cleanly separating business logic from concurrency retry logic. A worked example shows a task cancellation method before and after the refactor. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How can I avoid writing a messy while loop every time I use compareAndSet with AtomicReference in Kotlin? Factor the retry logic into a reusable extension function like testAndSet() that takes a condition and a new value, uses tailrec to retry on lost races, and returns the previous value instead of a boolean. This separates the concurrency retry mechanics from the business logic that decides whether and how to act on the previous state. _daily.dev surfaces concurrency patterns like this for developers refining their Kotlin threading code._ ### Why should a compareAndSet wrapper function return the previous value instead of a Boolean? Returning the previous value lets calling code react to what the state actually was before the update, such as checking if it was a specific subtype and acting on its properties, without a second read that could race. Realizing this was the key insight that made the testAndSet() function signature work cleanly. _Developers designing atomic state APIs can find similar concurrency design patterns on daily.dev._ ## Similar posts on daily.dev - [The Concurrency Bug That Quietly Breaks Fintech Apps](https://daily.dev/posts/the-concurrency-bug-that-quietly-breaks-fintech-apps-qqogb3pas) · Medium · 0 upvotes · 0 comments - [Recent JUnit Updates for Kotlin](https://daily.dev/posts/recent-junit-updates-for-kotlin-v9pujhmrj) · droidcon · 2 upvotes · 0 comments - [Finding Order in the Mayhem: A Novel Concurrency Testing Tool that Improved the Kotlin Compiler](https://daily.dev/posts/finding-order-in-the-mayhem-a-novel-concurrency-testing-tool-that-improved-the-kotlin-compiler-3rycypbau) · JetBrains · 0 upvotes · 0 comments - [Working in Kotlin: Waiting Around with Coroutines](https://daily.dev/posts/working-in-kotlin-waiting-around-with-coroutines-vjhedyeww) · Atomic Spin · 0 upvotes · 0 comments - [Features You Waited For So Long in Kotlin](https://daily.dev/posts/features-you-waited-for-so-long-in-kotlin-tdm4oviuu) · ProAndroidDev · 4 upvotes · 0 comments --- Tags: [#testing](https://daily.dev/tags/testing), [#kotlin](https://daily.dev/tags/kotlin) [View this post on daily.dev](https://daily.dev/posts/test-set-ahjohbn3v) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Test & Set","url":"https://daily.dev/posts/test-set-ahjohbn3v","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/test-set-ahjohbn3v"},"datePublished":"2026-08-23T12:21:22.585Z","dateModified":"2026-08-23T12:37:53.164Z","description":"A developer shares a reusable Kotlin extension function called testAndSet() that wraps the common compareAndSet() retry loop pattern used with AtomicReference....","image":"https://media.daily.dev/image/upload/s--2-1xRawN--/f_auto/v1722860399/public/Placeholder%2011","thumbnailUrl":"https://media.daily.dev/image/upload/s--2-1xRawN--/f_auto/v1722860399/public/Placeholder%2011","isAccessibleForFree":true,"articleSection":"Square","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Square","logo":"https://media.daily.dev/image/upload/s--QZPKZNBW--/f_auto,q_auto/v1787487457/logos/publicobject?_a=BAMAMicg0","url":"https://daily.dev/sources/publicobject"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/test-set-ahjohbn3v","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"testing,kotlin","timeRequired":"PT3M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Square","item":"https://daily.dev/sources/publicobject"},{"@type":"ListItem","position":3,"name":"Test & Set"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/test-set-ahjohbn3v#faq","mainEntity":[{"@type":"Question","name":"How can I avoid writing a messy while loop every time I use compareAndSet with AtomicReference in Kotlin?","acceptedAnswer":{"@type":"Answer","text":"Factor the retry logic into a reusable extension function like testAndSet() that takes a condition and a new value, uses tailrec to retry on lost races, and returns the previous value instead of a boolean. 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