Room: "Invalid return type for a type converter"

I have a Room database with the following entity:

data class NoteEntity(
    val startRef: BibleRef,
    val endRef: BibleRef,
    val content: String
)

I added the following type converter to store BibleRef fields as Ints:

class Converters {
    @TypeConverter
    fun bibleRefToInt(ref: BibleRef?): Int? {
        // performs conversion...
    }

    @TypeConverter
    fun bibleRefFromInt(refInt: Int?): BibleRef? {
        // performs conversion...
    }
}

I get the following build error (using kapt):

Invalid return type for a type converter. - Converters.bibleRefFromInt(java.lang.Integer)

BibleRef is a simple Kotlin data class.

data class BibleRef(
    val book: Int,
    val chapter: Int,
    val verse: Int
)

Solution

You appear to have not included an @TypeConverters annotation.

Noting the plural (defines the class or classes where the TypeConverters are) rather than the singular (defines a method/function as a TypeConverter).

In you case this would be :-

@TypeConverters(Converters::class)

And it is suggested that it is coded in the class that has the @Database annotation either immediately before or after the @Database annotation e.g.

@TypeConverters(Converters::class)
@Database(entities = [NoteEntity::class], version = 1)

This provides the greatest scope

Full code used to verify the above:-

BibleRef (unchanged)

data class BibleRef(
    val book: Int,
    val chapter: Int,
    val verse: Int
)

NoteEntity (see note)

@Entity
data class NoteEntity(
    @PrimaryKey
    val id: Long? = null,
    val startRef: BibleRef,
    val endRef: BibleRef,
    val content: String
)

Added id column as Primary Key as otherwise error: An entity must have at least 1 field annotated with @PrimaryKey public final class NoteEntity {
Added @Entity annotation otherwise error: Entity class must be annotated with @Entity public final class NoteEntity {

Converters (see notes)

class Converters {
    @TypeConverter
    fun bibleRefToInt(ref: BibleRef?): Int? {
        return 0
    }

    @TypeConverter
    fun bibleRefFromInt(ref: Int?): BibleRef? {
        return BibleRef(0,0,0)
    }
}

both changed to return a value of the appropriate type

TheDatabase added so that full compilation of Room code is undertaken and that generated java is generated.

@TypeConverters(Converters::class)
@Database(entities = [NoteEntity::class], version = 1)
abstract class TheDatabase: RoomDatabase() {
}

Note that no @Dao annotated classes have been used.

Build Log :-

Executing tasks: [:app:assembleDebug] in project E:\AndroidStudioApps\SO70425430KotlinRoomTypeConverterInvalidReturnType

> Task :app:preBuild UP-TO-DATE
> Task :app:preDebugBuild UP-TO-DATE
> Task :app:compileDebugAidl NO-SOURCE
> Task :app:compileDebugRenderscript NO-SOURCE
> Task :app:generateDebugBuildConfig UP-TO-DATE
> Task :app:checkDebugAarMetadata UP-TO-DATE
> Task :app:generateDebugResValues UP-TO-DATE
> Task :app:generateDebugResources UP-TO-DATE
> Task :app:mergeDebugResources UP-TO-DATE
> Task :app:createDebugCompatibleScreenManifests UP-TO-DATE
> Task :app:extractDeepLinksDebug UP-TO-DATE
> Task :app:processDebugMainManifest UP-TO-DATE
> Task :app:processDebugManifest UP-TO-DATE
> Task :app:processDebugManifestForPackage UP-TO-DATE
> Task :app:processDebugResources UP-TO-DATE
> Task :app:kaptGenerateStubsDebugKotlin
> Task :app:javaPreCompileDebug UP-TO-DATE
> Task :app:mergeDebugNativeDebugMetadata NO-SOURCE
> Task :app:mergeDebugShaders UP-TO-DATE
> Task :app:compileDebugShaders NO-SOURCE
> Task :app:generateDebugAssets UP-TO-DATE
> Task :app:mergeDebugAssets UP-TO-DATE
> Task :app:compressDebugAssets UP-TO-DATE
> Task :app:processDebugJavaRes NO-SOURCE
> Task :app:checkDebugDuplicateClasses UP-TO-DATE
> Task :app:desugarDebugFileDependencies UP-TO-DATE
> Task :app:mergeExtDexDebug UP-TO-DATE
> Task :app:mergeLibDexDebug UP-TO-DATE
> Task :app:mergeDebugJniLibFolders UP-TO-DATE
> Task :app:mergeDebugNativeLibs NO-SOURCE
> Task :app:stripDebugDebugSymbols NO-SOURCE
> Task :app:validateSigningDebug UP-TO-DATE
> Task :app:writeDebugAppMetadata UP-TO-DATE
> Task :app:writeDebugSigningConfigVersions UP-TO-DATE

> Task :app:kaptDebugKotlin
E:\AndroidStudioApps\SO70425430KotlinRoomTypeConverterInvalidReturnType\app\build\tmp\kapt3\stubs\debug\a\a\so70425430kotlinroomtypeconverterinvalidreturntype\TheDatabase.java:8: warning: Schema export directory is not provided to the annotation processor so we cannot export the schema. You can either provide `room.schemaLocation` annotation processor argument OR set exportSchema to false.
public abstract class TheDatabase extends androidx.room.RoomDatabase {
                ^

> Task :app:compileDebugKotlin
w: E:\AndroidStudioApps\SO70425430KotlinRoomTypeConverterInvalidReturnType\app\src\main\java\a\a\so70425430kotlinroomtypeconverterinvalidreturntype\Converters.kt: (9, 23): Parameter 'ref' is never used
w: E:\AndroidStudioApps\SO70425430KotlinRoomTypeConverterInvalidReturnType\app\src\main\java\a\a\so70425430kotlinroomtypeconverterinvalidreturntype\Converters.kt: (14, 25): Parameter 'ref' is never used

> Task :app:compileDebugJavaWithJavac
> Task :app:compileDebugSources
> Task :app:mergeDebugJavaResource UP-TO-DATE
> Task :app:dexBuilderDebug
> Task :app:mergeProjectDexDebug
> Task :app:packageDebug
> Task :app:assembleDebug

Deprecated Gradle features were used in this build, making it incompatible with Gradle 8.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/7.0.2/userguide/command_line_interface.html#sec:command_line_warnings

BUILD SUCCESSFUL in 2s
30 actionable tasks: 7 executed, 23 up-to-date

Build Analyzer results available