Room.databaseBuilder not running in Android Jetpack-CodePudding

I'm learning how to implement Room from Android jetpack into my android studio app but a message saying "not enough information to infer type variable T" keeps showing up when I try to use Room.databaseBuilder

this is my code:

    // if there is no instance, we create a new one
    synchronized(this){
        val instance = Room.databaseBuilder(
                context.applicationContext,
                UserDatabase::class.java,
                "user_database"
        ).build()
        INSTANCE = instance
        return instance
    }

I can add the surrounding function or the whole file if that helps. Thanks in advance!

CodePudding user response：

I'm not sure but this is the problem i faced as well and i solved it by downgrading 'implementation 'androidx.core:core-ktx:1.6.0' to 'implementation 'androidx.core:core-ktx:1.5.0' in the build.gradle file

CodePudding user response：

If you have for example:-

@Database(entities = [Objects::class], version = 1)
abstract class MyDatabase: RoomDatabase() {
    abstract fun getAllDao(): AllDAO

    companion object {
        var INSTANCE: MyDatabase? = null
         fun getInstance(context: Context): MyDatabase {
            synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    UserDatabase::class.java,
                    "user_database"
                ).build()
                INSTANCE = instance
                return instance
            }
        }
    }
}

The you get the not enough information to infer type variable T e.g. :-

Changing to use the same class then all is fine, e.g.

@Database(entities = [Objects::class], version = 1)
abstract class UserDatabase: RoomDatabase() { //<<<<<<<<<< CHANGED
    abstract fun getAllDao(): AllDAO

    companion object {
        var INSTANCE: UserDatabase? = null //<<<<<<<<<< CHANGED
         fun getInstance(context: Context): UserDatabase { //<<<<<<<<<< CHANGED
             if (INSTANCE == null) {
                 synchronized(this) {
                     val instance = Room.databaseBuilder(
                         context.applicationContext,
                         UserDatabase::class.java, //<<<<<<<<<< AS WAS/ MATCHES
                         "user_database"
                     )
                         .allowMainThreadQueries() /*<<<<<<<<<< ADDED FOR convenience brevity */
                         .build()
                     INSTANCE = instance
                     return instance
                 }
             }
             return INSTANCE!!
        }
    }
}

Note run on the main thread to just demonstrate that the above runs (see following example run)

As a test/proof consider :-

class MainActivity : AppCompatActivity() {
    lateinit var db1: UserDatabase
    lateinit var dao1: AllDAO

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        db1 = UserDatabase.getInstance(this) //<<<<<<<<<< GET INSTANCE
        dao1 = db1.getAllDao() 
        dao1.getObjects(listOf("nothing to get as no data has been added, just forcing open/create of database")) //<<<<<<<<< as per text USE/OPEN/CREATE Database(first use, once created database is opened as it persists)

        // existing app used ....

The result (ignore TheDatabase as existing code was used for the answer) :-