Question about Android studio error! Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

I'm making a simple note app with Android studio.

However, I try to run it on the emulator, but it keeps turning off and not running. There was no problem in the build window, so I faced a difficulty in how to solve it. Can I check the error in the stack trace I posted?

The stack trace of the error part is as follows

2020-07-06 00:56:05.456 8998-8998/comcutenote.cuteapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: comcutenote.cuteapp, PID: 8998
    java.lang.RuntimeException: Unable to start activity ComponentInfo{comcutenote.cuteapp/comcutenote.cuteapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at comcutenote.cuteapp.MainActivity.onCreate(MainActivity.java:29)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

The error partial code is as follows.

package comcutenote.cuteapp ;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

   static ArrayList<String> notes = new ArrayList<>();
   static ArrayAdapter arrayAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ListView listView = (ListView) findViewById(R.id.listView);

        notes.add("Example note");

        arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, notes);

        listView.setAdapter(arrayAdapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> parent, View view, int i, long l) {

                Intent intent = new Intent(getApplicationContext(), NoteEditorActivity.class);
                intent.putExtra("noteId", i);
                startActivity(intent);

            }
        });
    }
}

Can you tell me what is preventing the emulator from running?

I apologize in advance because I'm a serious Corinne, so there's a high possibility that you won't understand even if you tell me. But I'll try my best to solve it! Thank you!

android-studio nullpointerexception

2022-09-20 21:41

1 Answers

MainActivity.If the 29th line of java is listView.setAdapter(arrayAdapter); then

Caused by: java.lang.NullPointerException: 
    Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' 
        on a null object reference

    at comcutenote.cuteapp.MainActivity.onCreate(MainActivity.java:29)

There is a possibility that listView is null.

2022-09-20 21:41

If you have any answers or tips

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