Testing Room Database in Android using JUnit

import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity(tableName = "language")
data class Language(
    val languageName : String="",
    val experience : String=""
) {
    @PrimaryKey(autoGenerate = true)
    var id : Long=0
}

import androidx.room.Dao
import androidx.room.Insert
import androidx.room.Query

@Dao
interface LanguageDao {
    // Write two functions one for adding language to the database 
    // and another for retrieving all the items present in room db.
    @Insert
    suspend fun addLanguage(language: Language)

    @Query("SELECT * FROM language ORDER BY languageName DESC")
    suspend fun getAllLanguages(): List<Language>
}

import android.content.Context
import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase

@Database(entities = [Language::class] , version = 1)
abstract class LanguageDatabase : RoomDatabase() {

    // get reference of the dao interface that we just created
    abstract fun getLanguageDao() : LanguageDao

    companion object{
        private const val DB_NAME = "Language-Database.db"
        
        // Get reference of the LanguageDatabase and assign it null value
        @Volatile
        private var instance : LanguageDatabase? = null
        private val LOCK = Any()

        // create an operator fun which has context as a parameter
        // assign value to the instance variable
        operator fun invoke(context: Context) = instance ?: synchronized(LOCK){
            instance ?: buildDatabase(context).also{
                instance = it
            }
        }
        // create a buildDatabase function assign the required values
        private fun buildDatabase(context: Context) = Room.databaseBuilder(
            context.applicationContext,
            LanguageDatabase::class.java,
            DB_NAME
        ).fallbackToDestructiveMigration().build()
    }
}

import android.content.Context
import androidx.room.Room
import androidx.test.core.app.ApplicationProvider
import androidx.test.ext.junit.runners.AndroidJUnit4
import com.google.common.truth.Truth.assertThat
import junit.framework.TestCase
import kotlinx.coroutines.runBlocking
import org.junit.*
import org.junit.runner.RunWith

@RunWith(AndroidJUnit4::class) // Annotate with @RunWith
class LanguageDatabaseTest : TestCase() {
    // get reference to the LanguageDatabase and LanguageDao class
    private lateinit var db: LanguageDatabase
    private lateinit var dao: LanguageDao

    // Override function setUp() and annotate it with @Before
    // this function will be called at first when this test class is called
    @Before
    public override fun setUp() {
        // get context -- since this is an instrumental test it requires
        // context from the running application
        val context = ApplicationProvider.getApplicationContext<Context>()
        // initialize the db and dao variable 
        db = Room.inMemoryDatabaseBuilder(context, LanguageDatabase::class.java).build()
        dao = db.getLanguageDao()
    }

    // Override function closeDb() and annotate it with @After
    // this function will be called at last when this test class is called
    @After
    fun closeDb() {
        db.close()
    }

    // create a test function and annotate it with @Test 
    // here we are first adding an item to the db and then checking if that item 
    // is present in the db -- if the item is present then our test cases pass
    @Test
    fun writeAndReadLanguage() = runBlocking {
        val language = Language("Java", "2 Years")
        dao.addLanguage(language)
        val languages = dao.getAllLanguages()
        assertThat(languages.contains(language)).isTrue()
    }
}