致命异常 java.lang.NullPointerException：null 不能转换为非 null 类型 kotlin.String

Question

我尝试制作我的第一个应用程序，但我收到了错误消息。 错误信息是：

2020-09-17 11:18:17.846 30034-30132/com.ibrahimkiceci.simplynoteapp W/i.simplynoteap: Accessing hidden method Lsun/misc/Unsafe;->getInt(Ljava/lang/Object;J)I (greylist, linking, allowed)

2020-09-17 11:18:18.017 30034-30034/com.ibrahimkiceci.simplynoteapp D/AndroidRuntime：关闭 VM 2020-09-17 11:18:18.020 30034-30034-30034-30034-30034-30034-30034-com.例外：主进程：com.ibrahimkiceci.simplynoteapp，PID：30034 java.lang.NullPointerException：在 com.ibrahimkiceci.simplynoteapp.ListViewActivity$getDataFromFirestore$1.onEvent(ListViewActivity.kt 114) 在 com.ibrahimkiceci.simplynoteapp.ListViewActivity$getDataFromFirestore$1.onEvent(ListViewActivity.kt:19) 在 com.google.firebase.firestore.Query.lambda$addSnapshotListenerInternal$2(Query.java:1142) 在 com.google.firebase .firestore.Query$$Lambda$3.onEvent(Unknown Source:6) at com.google.firebase.firestore.core.AsyncEventListener.lambda$onEvent$0(AsyncEventListener.java:42) at com.google.firebase.firestore.core .AsyncEventListener$$Lambda$1.run(Unknown Source:6) at android.os.Handler.handleCallback(Handler.java:883) at android.os.Handler.di spatchMessage(Handler.java:100) at android.os.Looper.loop(Looper.java:214) at android.app.ActivityThread.main(ActivityThread.java:7356) at java.lang.reflect.Method.invoke(Native)方法）在 com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 2020-09-17 11:18 :18.086 30034-30034/com.ibrahimkiceci.simplynoteapp I/Process: 发送信号。 PID：30034 SIG：9

当我点击我的登录按钮时，应用程序关闭，我分享了代码屏幕，

我该如何解决？ 谢谢！

class ListViewActivity : AppCompatActivity() { private lateinit var auth: FirebaseAuth private lateinit var db : FirebaseFirestore

var titleTextFromFB : ArrayList<String> = ArrayList()
var imageFromFB : ArrayList<String> = ArrayList()
var adapter: NoteAdapter? = null


override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_list_view)

    auth = FirebaseAuth.getInstance()
    db = FirebaseFirestore.getInstance()

    getDataFromFirestore()

    // recyclerview

    var layoutManager = LinearLayoutManager(this)
    recyclerView.layoutManager = layoutManager
    adapter = NoteAdapter(titleTextFromFB, imageFromFB)

    recyclerView.adapter = adapter

}

override fun onCreateOptionsMenu(menu: Menu?): Boolean {

    val menuInflater = menuInflater
    menuInflater.inflate(R.menu.add_note, menu)

    return super.onCreateOptionsMenu(menu)
}

override fun onOptionsItemSelected(item: MenuItem): Boolean {

    if (item.itemId == R.id.add_note) {
        // Take Notes Activity
        val intent = Intent(applicationContext, TakeNotesActivity::class.java)
        startActivity(intent)

    } else if (item.itemId == R.id.log_out) {

        val alert = AlertDialog.Builder(this)

        alert.setTitle("Log out")
        alert.setMessage("Are you sure to logout from the app ?")
        alert.setPositiveButton("Yes") {dialog, which ->

            auth.signOut()
            val intent = Intent(applicationContext, MainActivity::class.java)
            startActivity(intent)
            finish()
        }

        alert.setNegativeButton("No") {dialog, which ->

        }

        alert.show()


    }


    return super.onOptionsItemSelected(item)
}

// get data from firestore

fun getDataFromFirestore() {

    db.collection("Notes").orderBy("date", Query.Direction.DESCENDING).addSnapshotListener { snapshot, exception ->

        if (exception != null) {

            // If there is a error ,

            Toast.makeText(applicationContext, exception.localizedMessage.toString(), Toast.LENGTH_LONG).show()


        } else {

            if (snapshot != null) {

                if (!snapshot.isEmpty) {

                    val documents = snapshot.documents
                    for (document in documents) {

                        val userEmail = document.get("userEmail") as String
                        val noteTitle = document.get("noteTitle") as String
                        val yourNote = document.get("yourNote") as String
                        val downloadUrl = document.get("downloadUrl") as String
                        val timestamp = document.get("date") as Timestamp
                        val date = timestamp.toDate()

                        titleTextFromFB.add(noteTitle)
                        imageFromFB.add(downloadUrl)

                        adapter!!.notifyDataSetChanged()




                    }
                }
            }


        }


      }



  }




}

Answer 1

据我了解，您的错误是由此代码块生成的：

val userEmail = document.get("userEmail") as String
val noteTitle = document.get("noteTitle") as String
val yourNote = document.get("yourNote") as String
val downloadUrl = document.get("downloadUrl") as String

这里document一个变量是 null，它不能在 kotlin 中转换为不可为 null 的String类型。

如果您不确定哪些字段可以为空，请编写如下代码：

val userEmail : String? = document.get("userEmail") as? String
val noteTitle : String?  = document.get("noteTitle") as? String
val yourNote : String? = document.get("yourNote") as? String
val downloadUrl : String? = document.get("downloadUrl") as? String

为了更清楚，请检查 kotlin 文档中的Safe(nullable)-Cast运算符

Answer 2

在你的 for 循环中

for (document in documents) {

                    val userEmail = document.get("userEmail") as String
                    val noteTitle = document.get("noteTitle") as String
                    val yourNote = document.get("yourNote") as String
                    val downloadUrl = document.get("downloadUrl") as String
                    val timestamp = document.get("date") as Timestamp
                    val date = timestamp.toDate()

                    titleTextFromFB.add(noteTitle)
                    imageFromFB.add(downloadUrl)

                    adapter!!.notifyDataSetChanged()




                }

一个（或多个）字段为空，而您正试图将它们转换为非空 String 对象！ 所以在这种情况下，kotlin 会抛出一个运行时异常。 你应该把它转换成字符串吗？ 而不是字符串：像这样：

val field = document.get("field") as String?

或检查其可空性：

val field = if (document.get("field") != null) document.get("field") as String
    else ""

Answer 3

当我删除并重新安装我的数据库时，问题解决了。