kotlin 中的“NullPointerException：null 不能转换为非空类型错误”

Question

 var notify = ArrayList<String>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main_user)
        btn_list2.setOnClickListener {

            val sharedPreferences1 = getSharedPreferences("id", Context.MODE_PRIVATE)
            val documentid: String? = sharedPreferences1.getString("id","null")
            val c = FirebaseFirestore.getInstance()
            val d = c.collection("applicationForm").document(documentid.toString()).get()
                .addOnSuccessListener { document ->

                    notify = document?.get("notifyTo") as ArrayList<String>

                    var str1 = notify.joinToString()
                    Toast.makeText(applicationContext,str1,Toast.LENGTH_SHORT).show()

}}}

Here I get error in the line notify = document?.get("notifyTo") as ArrayList<String> .

This is my logcat details

java.lang.NullPointerException: null cannot be cast to non-null type java.util.ArrayList<kotlin.String>
        at com.example.bloodbankcompany.MainActivityUser.onCreate$lambda-4$lambda-3(MainActivityUser.kt:47)
        at com.example.bloodbankcompany.MainActivityUser.lambda$cGlrfLFSOO25IeEAacXMuz6Tzx0(Unknown Source:0)`. 

Please can anyone help. Here I am trying to read a array document from firestore.

Answer 1

First of all, .addOnSuccessListener returns the result, not the value so it will obviously cause an exception.

    var documents: ArrayList<String> = arrayListOf()

    c.collection("applicationForm").document(documentid.toString()).get()
                .addOnSuccessListener { result ->
                documents = result.value
    }

also check if result.values is != null , get the value like this, also at the end check if you are mapping the collection correctly.

Answer 2

You are correctly gating document?.get(...) but you are casting the result to an ArrayList.

Since either document is null OR there is no "notifyTo" key in your document result, you end up basically doing null as ArrayList<String> . Hence the error.

To stop getting the crash, you need to do document.get("notifyTo") as? ArrayList<String> document.get("notifyTo") as? ArrayList<String>

But really want you want to make sure of is that "notifyTo" exists inside your document so that you no longer get a null return value