[英]How to instantiate an abstract class in Kotlin?
我想出了以下问题:
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)
// ...
}
}
房间的数据库:
@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,我是这样做的:
db = AppDataBase.getDataBase(view.getContext());
如何在 Kotlin 中获得相同的结果?
使用单例(在 Kotlin object
)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 {
private const val DB_NAME: String = "DataBaseUser"
private lateinit var dbInstance: DataBase
fun getDatabase(context: Context): DataBase {
if (!this::dbInstance.isInitialized) {
dbInstance = Room.databaseBuilder(
context.applicationContext,
DataBase::class.java,
DB_NAME
).build()
}
return dbInstance
}
}
用法:
val db = DatabaseProvider.getDatabase(view.getContext())
要在 Kotlin 中实例化抽象类,请使用object: <your class>
。 例子:
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.
}
}
}
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.
}
}
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