Null object reference error on findViewById() even when invoked after setContentView()

Question

I am trying to get an instance of a SeekBar using the findViewById() and attaching a listener to it.

The code looks like this:

SeekBar seek_bar = findViewById(R.id.audio_volume);
seek_bar.setOnSeekBarChangeListener(listener);

And the listener:

private SeekBar.OnSeekBarChangeListener listener = new SeekBar.OnSeekBarChangeListener() {
        @Override
        public void onProgressChanged(SeekBar seekBar, int i, boolean b) {
            log.d("YO HERE");
        }

        @Override
        public void onStartTrackingTouch(SeekBar seekBar) {

        }

        @Override
        public void onStopTrackingTouch(SeekBar seekBar) {

        }
    };

Followed this SO post , and invoked the findViewById inside the onCreate method after these two lines:

super.onCreate(savedInstanceState);
setContentView(R.layout.main);

But the problem persists. I am still getting:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.SeekBar.setOnSeekBarChangeListener(android.widget.SeekBar$OnSeekBarChangeListener)' on a null object reference

And the app fails to open. How can I solve this issue? Also it would be very helpful if the process of debugging a null object reference error is shared.

Layout xml:

    <?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen
  xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
  <ListPreference
   android:key="PREF_MIN_REC"
   android:title="@string/min_rec"
   android:summary="@string/min_rec_desc"
   android:entries="@array/min_rec_options"
   android:entryValues="@array/min_rec_values"
   android:dialogTitle="@string/min_rec"
   android:defaultValue="15"
  />
  <ListPreference
   android:key="PREF_AUDIO_SOURCE"
   android:title="@string/audio_source"
   android:summary="@string/audio_source_desc"
   android:entries="@array/audio_source_options"
   android:entryValues="@array/audio_source_values"
   android:dialogTitle="@string/audio_source"
   android:defaultValue="4"
  />
  <ListPreference
    android:key="PREF_AUDIO_FORMAT"
    android:title="@string/audio_format"
    android:summary="@string/audio_format_desc"
    android:entries="@array/audio_format_options"
    android:entryValues="@array/audio_format_values"
    android:dialogTitle="@string/audio_format"
    android:defaultValue="1"
  />
    <org.anasthase.androidseekbarpreference.SeekBarPreference
        android:id="@+id/volume"
        android:key="PREF_MIC_VOLUME"
        android:title="@string/mic_volume"
        android:summary="@string/mic_volume_desc"
        app:maxValue="100"
        app:minValue="1"
        app:stepValue="1"
        android:defaultValue="20"
        app:format="%.1f"
        app:displayDividerValue="5"/>

  <org.anasthase.androidseekbarpreference.SeekBarPreference
          android:id="@+id/audio_volume"
          android:key="PREF_AUDIO_VOLUME"
          android:title="@string/audio_volume"
          android:summary="@string/audio_volume_desc"
          app:maxValue="100"
          app:minValue="1"
          app:stepValue="1"
          android:defaultValue="20"
          app:format="%.1f"
          app:displayDividerValue="5"/>


  <EditTextPreference
    android:key="PREF_URL_SUBDOMAIN"
    android:title="@string/server_subdomain"
    android:summary="@string/server_subdomain_desc"
    android:dialogTitle="@string/server_subdomain"
    android:defaultValue=".telforce.biz"
  />
  <ListPreference
      android:key="PREF_UPLOAD_NETWORK"
      android:title="@string/upload_network"
      android:summary="@string/upload_network_desc"
      android:entries="@array/upload_network_options"
      android:entryValues="@array/upload_network_values"
      android:dialogTitle="@string/upload_network"
      android:defaultValue="1"
      />

  <CheckBoxPreference
    android:key="save_term_enable_key"
    android:title="@string/save_term_check_title"
    android:summary="@string/save_term_check_summary"/>
  <EditTextPreference
    android:key="save_term_key"
    android:title="@string/save_term_title"
    android:summary="@string/save_term_summary"
    android:dependency="save_term_enable_key"
    android:defaultValue="@string/save_term_default_value"
    android:dialogTitle="@string/save_term_dialog"/>

</PreferenceScreen>

Answer 1

In your XML your audio_volume view belongs to class org.anasthase.androidseekbarpreference.SeekBarPreference

In your Java code, it is SeekBar only, so if it is not crashing, I assume Seekbar extends SeekBarPreference.

Ignore the casting comments and responses since it's not needed since last year if you're using the latest versions of java, android sdk and target api.

The null pointer exception means the seekbar was not found on the layout. Check if you have more than 1 xml with the same name (for different screen resolutions).

Answer 2

do this way if its preference fragment:

SeekBarPreference skp = (SeekBarPreference) findPreference("PREF_AUDIO_VOLUME"); 

Cheers ;)