Passing arrayAdapter to ListViewAdapter in android - android-studio

Question

Am having problem while passing my arrayAdapter the ListViewAdapter in android using kotlin.
The error message java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference indicates that I am trying to call a method on an object reference which is null.
Here is my mainActivity:
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.ArrayAdapter
import android.widget.ListView
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val lv = findViewById<ListView>(R.id.myList)
val array = arrayOf("test", "test", "test", "test", "test")
val myAdapter: ArrayAdapter<String> =
ArrayAdapter(this, android.R.layout.simple_list_item_1, array)
lv.adapter = myAdapter
}
}
Here is the Layout file:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/relativeLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingHorizontal="20dp"
tools:context=".MainActivity">
<ListView
android:id="#+id/myList"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_marginStart="20dp"
android:layout_marginEnd="20dp" />
</LinearLayout>
And here is the error that I got
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.myapplication5.MainActivity.onCreate(MainActivity.kt:16)
at android.app.Activity.performCreate(Activity.java:7994)
at android.app.Activity.performCreate(Activity.java:7978)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:223) 
at android.app.ActivityThread.main(ActivityThread.java:7656) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2022-12-25 14:33:52.075 14422-14438/com.example.myapplication5 W/System: A resource failed to call close.
Debug shows that myAdapter contains corretly the array values

Answer 1

You are passing ArrayAdapter where in error you can check it needs ListAdapter.
Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)

Answer 2

You have to call setContentView(<Name of your layout file>) before accessing any views. See working example (in my case the name of the layout file is activity_main.xml):
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val lv = findViewById<ListView>(R.id.myList)
val array = arrayOf("test", "test", "test", "test", "test")
val myAdapter: ArrayAdapter<String> =
ArrayAdapter(this, android.R.layout.simple_list_item_1, array)
lv.adapter = myAdapter
}
}