{"id":187,"date":"2026-09-24T15:47:00","date_gmt":"2026-09-24T13:47:00","guid":{"rendered":"https:\/\/grindloop.io\/blog\/?p=187"},"modified":"2026-09-27T01:20:41","modified_gmt":"2026-09-26T23:20:41","slug":"kotlin-data-class-equals-ignores-parent-class","status":"publish","type":"post","link":"https:\/\/grindloop.ai\/blog\/kotlin-data-class-equals-ignores-parent-class\/","title":{"rendered":"Why Your Kotlin Data Class’s equals() Ignores the Parent Class"},"content":{"rendered":"

A bookmarks app merges a cached list with a freshly synced one and calls .distinct()<\/code>. Two bookmarks with different server IDs but the same title and URL collapse into one. A record disappears. Bookmark<\/code> is a data class. Its server ID lives on a base class. What’s wrong?<\/p><\/blockquote>\n\n\n

open class RemoteEntity {\n    var remoteId: String = \"\"\n        private set\n    var syncedAt: Long = System.currentTimeMillis()\n        private set\n\n    fun markSynced(id: String) {\n        remoteId = id\n        syncedAt = System.currentTimeMillis()\n    }\n}\n\ndata class Bookmark(\n    val title: String,\n    val url: String\n) : RemoteEntity()<\/pre>\n\n\n\n
val cachedCopy = Bookmark(title = \"Kotlin Docs\", url = \"https:\/\/kotlinlang.org\").apply { markSynced(\"rec_101\") }\nval freshCopy  = Bookmark(title = \"Kotlin Docs\", url = \"https:\/\/kotlinlang.org\").apply { markSynced(\"rec_204\") }\n\ncachedCopy == freshCopy                       \/\/ true\ncachedCopy.remoteId == freshCopy.remoteId     \/\/ false<\/pre>\n\n\n
The Kotlin data class equals check ignores a parent class’s properties. The compiler generates equals()<\/code>, hashCode()<\/code>, toString()<\/code> and copy()<\/code> from the data class’s own primary constructor only. Bookmark<\/code>‘s constructor has title<\/code> and url<\/code>. remoteId<\/code> lives on RemoteEntity<\/code>, so the generated equals()<\/code> never sees it. Two bookmarks with different IDs but the same title and URL compare equal. They collapse into one in a Set<\/code> or after .distinct()<\/code>. This is documented, intentional design, not a compiler bug. The fix is to override equals()<\/code> and hashCode()<\/code> by hand, including remoteId<\/code>. There’s a second trap in that fix. Both functions must use exactly the same fields. A hashCode()<\/code> that includes a field equals()<\/code> ignores breaks hash-based collections.<\/p>\n\n
Bug 1: the Kotlin data class equals check stops at its own constructor<\/h2>\n\n
Kotlin’s data classes documentation<\/a> states the scope. The compiler only uses properties defined in the primary constructor for the generated functions. That rule is usually read as being about properties in the class body. It also excludes everything on a superclass.<\/p>\n\n
The design goes back to KEEP-0031<\/a>, the proposal that let data classes extend other classes. It’s marked stable since Kotlin 1.1. It covers the base class’s primary constructor. Those properties don’t take part in componentN<\/code> and the other special functions. In April 2025, a developer filed KEEP issue 419<\/a>. It called this an incorrect equals()<\/code> for data classes with inheritance. The issue is closed. In an interview, “it’s a compiler bug” is the wrong answer.<\/p>\n\n
The damage shows up when merging lists.<\/p>\n\n\n
val merged = (cachedBookmarks + freshBookmarks).distinct()\n\/\/ two bookmarks with different remoteId but the same title\/url\n\/\/ collapse into a single entry, and one of them is gone<\/pre>\n\n\n
Bug 2: the obvious fix hashes a field equals() doesn’t check<\/h2>\n\n
A common first fix is to override equals()<\/code> and hashCode()<\/code> by hand. The mistake shows up in hashCode()<\/code>.<\/p>\n\n\n
\/\/ Common wrong answer: fixes equals(), breaks the equals\/hashCode contract\ndata class Bookmark(\n    val title: String,\n    val url: String\n) : RemoteEntity() {\n\n    override fun equals(other: Any?): Boolean {\n        if (this === other) return true\n        if (other !is Bookmark) return false\n        return remoteId == other.remoteId && title == other.title && url == other.url\n    }\n\n    override fun hashCode(): Int {\n        return Objects.hash(remoteId, title, url, syncedAt)\n    }\n}<\/pre>\n\n\n
syncedAt<\/code> ends up in hashCode()<\/code> because it’s on the object. More fields feels safer. But equals()<\/code> never checks it. Kotlin’s Any.hashCode()<\/code> contract<\/a> requires the hash to stay the same as long as nothing used in equals()<\/code> changes. Here, changing syncedAt<\/code> changes the hash even though equality didn’t change.<\/p>\n\n\n
val bookmark = Bookmark(title = \"Kotlin Docs\", url = \"https:\/\/kotlinlang.org\").apply { markSynced(\"rec_101\") }\nval cache = hashSetOf(bookmark)\n\nbookmark.markSynced(\"rec_101\")   \/\/ re-synced later; same id, syncedAt changed\n\ncache.contains(bookmark)         \/\/ false<\/pre>\n\n\n
The object is still in cache<\/code>. HashSet<\/code> can’t find it, because contains()<\/code> recomputes the hash to pick a bucket. The new hash points to a different bucket from the one the object was filed in.<\/p>\n\n
The fix<\/h2>\n\n\n
data class Bookmark(\n    val title: String,\n    val url: String\n) : RemoteEntity() {\n\n    override fun equals(other: Any?): Boolean {\n        if (this === other) return true\n        if (other !is Bookmark) return false\n        return remoteId == other.remoteId && title == other.title && url == other.url\n    }\n\n    override fun hashCode(): Int = Objects.hash(remoteId, title, url)\n}<\/pre>\n\n\n
hashCode()<\/code> now uses exactly the fields equals()<\/code> checks. syncedAt<\/code> stays on the object for display and sync logic, with no say in identity. One caution remains. remoteId<\/code> is still mutable through markSynced()<\/code>. So don’t put a bookmark in a hash-based collection until it has its final ID. If you can, pass the ID into the constructor and make it a val<\/code>. Then the generated functions handle it and the problem disappears.<\/p>\n\n
The rule to remember<\/h2>\n\n
A data class’s generated functions only see its own primary constructor. If you override equals()<\/code> and hashCode()<\/code>, build both from the same fields. Keep those fields stable while the object is in a hash-based collection.<\/strong><\/p>\n\n