Passing arrayAdapter to ListViewAdapter in android-CodePudding

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

CodePudding user response：

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)

CodePudding user response：

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
    }
}