kotlin - GrindLoop: Android & iOS Interview Prep

String.format Locale Bug: Why “1234,50” Throws NumberFormatException on Android

October 9, 2026 by Arsam

Calculator on euro banknotes, illustrating a String.format locale bug in an Android checkout amount

A String.format locale bug turns 1234.50 into 1234,50 or Arabic digits, so toDouble() throws NumberFormatException. Runnable repro, fix and interview answer.

Does Taking a Hint in a Coding Interview Count Against You?

October 9, 2026 by Hass

Two light bulbs on a table, one lit, for a post on taking a hint in a coding interview

A hint in a coding interview costs points by its size. How the hint ladder works, when to ask for help and a full script for asking and recovering.

Kotlin Mutex withLock Deadlock: Why a Nested withLock Hangs Forever

October 8, 2026 by Arsam

Rusty padlock hanging from a chain, illustrating a Mutex withLock deadlock in Kotlin coroutines

A Mutex withLock deadlock comes from a non-reentrant Mutex locked twice, and it hangs with no ANR or exception. A runnable repro, the fix and wrong answers.

LaunchedEffect Stale Value: Why LaunchedEffect(Unit) Calls an Old Callback

October 7, 2026 by Arsam

Hourglass running out, like a question timer hit by a LaunchedEffect stale value bug

A LaunchedEffect stale value comes from LaunchedEffect(Unit) keeping its first lambda and a captured String. Why keying on the callback fails, and the fix.

StateFlow Not Emitting When You Add to a MutableList Inside update {}

October 6, 2026 by Arsam

Miniature shopping cart and paper bag, illustrating a StateFlow not emitting when a cart item is added

StateFlow not emitting after you add to a MutableList? copy() shares the list so states compare equal, and update {} retries repeat the add. Repro and fix.

Why a Non-Null Kotlin Property Is Null When the Base Class Init Calls an Open Function

October 5, 2026 by Arsam

Syntax-highlighted code on a dark screen, illustrating why a non-null Kotlin property is null during base class init

A non-null Kotlin property is null when a base class init calls an open function. Why the subclass fields are unset, why it compiles, and the fix.

Why Reading the Response Body in an OkHttp Interceptor Throws IllegalStateException: closed

October 4, 2026 by Arsam

Close-up of exception-handling code on a screen, illustrating IllegalStateException: closed in an OkHttp interceptor

Reading the response body in an OkHttp interceptor causes IllegalStateException: closed. The two stacked causes, the peekBody fix and the interview answer.

Why a Private Constructor Doesn’t Stop Kotlin Data Class copy()

October 3, 2026 by Arsam

Padlock on a blue metal door, illustrating how data class copy gets around a private constructor

A private constructor doesn’t stop the Kotlin data class copy() from creating invalid objects. The two causes, what Kotlin 2.5 changes and the fix.

Why a Listener That Removes Itself Throws ConcurrentModificationException

October 2, 2026 by Arsam

For loop in syntax-highlighted code on a dark screen, illustrating a listener loop that throws ConcurrentModificationException

Unregistering a listener inside its own callback throws ConcurrentModificationException or skips listeners. Why, JVM vs Android, and the snapshot fix.

Why Offline Sync Duplicates Records After a Retry

October 1, 2026 by Arsam

A smartphone lying face up on a white table, illustrating offline sync duplicates after a retry

Offline sync duplicates records when a retried POST can’t prove it’s a retry. The two stacked bugs, a runnable Kotlin repro, the fix and the interview answer.