Question

我提出了以下问题：

我的主要活动代码：

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // ...
        // ERROR: Cannot create instance of an abstract class
        var db = DataBase().getDataBase(this)
        // ...
    }
}

Room的数据库：

@Database(entities = arrayOf(DataBaseUser::class), version = 1)
abstract class DataBase : RoomDatabase() {

    val DB_NAME : String = "DataBaseUser"
    private lateinit var INSTANCE : DataBase

    fun getDataBase(context: Context): DataBase {
        if (INSTANCE == null){
            INSTANCE = Room.databaseBuilder(context.applicationContext,DataBase::class.java,DB_NAME).build()
        }
        return INSTANCE
    }

    abstract fun getUserDao(): DataBaseUserDao
}

我无法在Kotlin中实例化该类。在Java中，我这样做了：

db = AppDataBase.getDataBase(view.getContext());

如何在Kotlin中获得相同的结果？

Answer 1

使用单例（在Kotlin object中）来保存Room数据库实例：

@Database(entities = arrayOf(DataBaseUser::class), version = 1)
abstract class DataBase : RoomDatabase() {
    abstract fun getUserDao(): DataBaseUserDao
}

object DatabaseProvider {
    val DB_NAME : String = "DataBaseUser"
    private lateinit var INSTANCE : DataBase

    fun getDataBase(context: Context): DataBase {
        if (INSTANCE == null) {
            INSTANCE = Room.databaseBuilder(context.applicationContext,DataBase::class.java,DB_NAME).build()
        }

        return INSTANCE
    }
}

用法：

val db = DatabaseProvider.getDataBase(view.getContext())

Answer 2

要在Kotlin中实例化抽象类，请使用object: <your class>。例如：

abstract class AbstractTest {
    abstract fun someFun(): Int
}

fun main(args: Array<String>) {
    val astractInstance = object : AbstractTest() {
        override fun someFun(): Int {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }
    }
}

Answer 3

您可以在下面使用

var db = object: DataBase(){
    override fun getUserDao(): DataBase{
    TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
    }
}

如何在Kotlin中实例化一个抽象类？

3 个答案: