我是Kotlin的新手，正在尝试学习如何进行翻新以获取数据并将其存储到Room DB中。但是，一旦我开始执行该过程的活动，我就会得到NullPointerException。我的猜测是发生错误是因为访问时我数据库的内容为Null。
If my code snippets aren't enough, here's a link to the GitHub repository from the mini-project I'm working on: https://github.com/Engin92/Dog_Breeds/tree/RoomDatabase
这是我完整的错误列表：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.dogbreeds, PID: 14803
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.dogbreeds/com.example.Breedlist.activity.DetailedViewActivity}: kotlin.KotlinNullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3782)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3961)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:91)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:149)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:103)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2386)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:213)
at android.app.ActivityThread.main(ActivityThread.java:8178)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1101)
Caused by: kotlin.KotlinNullPointerException
at com.example.Breedlist.activity.DetailedViewActivityRepository.getBreeds(DetailedViewActivityRepository.kt:23)
at com.example.Breedlist.activity.DetailedViewActivityViewModel.getAllBreedList(DetailedViewActivityViewModel.kt:23)
at com.example.Breedlist.activity.DetailedViewActivity.onCreate(DetailedViewActivity.kt:42)
at android.app.Activity.performCreate(Activity.java:8086)
at android.app.Activity.performCreate(Activity.java:8074)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1313)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3755)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3961)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:91)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:149)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:103)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2386)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:213)
at android.app.ActivityThread.main(ActivityThread.java:8178)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1101)
DetailsViewActivityRepository中的getBreeds函数：
fun getBreeds() : LiveData<List<CurrentBreedResponseItem>>
{
return RoomViewModel.database!!.currentBreedDao().getAllBreeds()
}
DetailsViewActivityViewModel中的getAllBreedList函数
fun getAllBreedList(): LiveData<List<CurrentBreedResponseItem>>
{
return detailedViewActivityRepository.getBreeds()
}
详细视图活动
class DetailedViewActivity : AppCompatActivity() {
lateinit var breedRecyclerView: RecyclerView
lateinit var detailedViewActivityViewModel: DetailedViewActivityViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_detailed_view)
breedRecyclerView = findViewById(R.id.breedRecyclerView)
detailedViewActivityViewModel = ViewModelProviders.of(this).get(
DetailedViewActivityViewModel::class.java)
if(isOnline(this))
{
detailedViewActivityViewModel.getBreedsFromAPIAndStore()
}
else
{
Toast.makeText(this,"No internet connection. Showing cached list!",Toast.LENGTH_LONG).show()
}
detailedViewActivityViewModel.getAllBreedList().observe(this, Observer<List<CurrentBreedResponseItem>> { breedList ->
Log.e(MainActivity::class.java.simpleName,breedList.toString())
setUpBreedRecyclerView(breedList!!)
})
}
fun isOnline(context: Context): Boolean {
val connectivityManager =
context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
if (connectivityManager != null) {
val capabilities =
connectivityManager.getNetworkCapabilities(connectivityManager.activeNetwork)
if (capabilities != null) {
if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR)) {
Log.i("Internet", "NetworkCapabilities.TRANSPORT_CELLULAR")
return true
} else if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_WIFI)) {
Log.i("Internet", "NetworkCapabilities.TRANSPORT_WIFI")
return true
} else if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET)) {
Log.i("Internet", "NetworkCapabilities.TRANSPORT_ETHERNET")
return true
}
}
}
return false
}
fun setUpBreedRecyclerView(breeds : List<CurrentBreedResponseItem>)
{
val breedRecyclerViewAdapter =
BreedRecyclerViewAdapter(this, breeds)
breedRecyclerView.adapter = breedRecyclerViewAdapter
breedRecyclerView.layoutManager = GridLayoutManager(this,2)
breedRecyclerView.setHasFixedSize(true)
}
}
类RoomViewModel，我在其中构建数据库
class RoomViewModel : Application() {
companion object {
var database: BreedDatabase? = null
}
override fun onCreate() {
super.onCreate()
database = Room.databaseBuilder(applicationContext, BreedDatabase::class.java, "breed_db").fallbackToDestructiveMigration().build()
}
}
