Android Studio Method can not be called in PreferenceActivity

Question

I want to call the this method (the method is placed in the mainActivity):

public void setTextView(String[] countryArray, String column){
    TextView myAwesomeTextView;
    String textViewID;

    for(int i=0; i<=25; i++) {
        if(column.equals("left")){
            textViewID = "textViewColumn1_" + i;
        } else {
            textViewID = "textViewColumn2_" + i;
        }
        int resID = getResources().getIdentifier(textViewID, "id", getPackageName());
        myAwesomeTextView = (TextView) findViewById(resID);
        myAwesomeTextView.setText(countryArray[i]);
    }
}

If I call the method in my MainActivity their is no problem. But I want to call this in my AppCompatPreferenceActivity when the user change the settings of the app. But then the app crashes:

@Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
    // handle the preference change here
    mainActivity.setTextView(usa, "left");
}

Android Monitor:

E/AndroidRuntime: FATAL EXCEPTION: main
                                                                          Process: com.larsus.test.testproject, PID: 56871
                                                                          java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                                                                              at android.content.ContextWrapper.getResources(ContextWrapper.java:85)
                                                                              at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:74)
                                                                              at android.support.v7.app.AppCompatActivity.getResources(AppCompatActivity.java:548)
                                                                              at com.larsus.test.testproject.ScrollingActivity.setTextView(ScrollingActivity.java:126)
                                                                              at com.larsus.test.testproject.AppCompatPreferenceActivity.onSharedPreferenceChanged(AppCompatPreferenceActivity.java:50)
                                                                              at android.app.SharedPreferencesImpl$EditorImpl.notifyListeners(SharedPreferencesImpl.java:476)
                                                                              at android.app.SharedPreferencesImpl$EditorImpl.apply(SharedPreferencesImpl.java:384)
                                                                              at android.preference.Preference.tryCommit(Preference.java:1433)
                                                                              at android.preference.Preference.persistString(Preference.java:1466)
                                                                              at android.preference.ListPreference.setValue(ListPreference.java:147)
                                                                              at android.preference.ListPreference.onDialogClosed(ListPreference.java:282)
                                                                              at android.preference.DialogPreference.onDismiss(DialogPreference.java:391)
                                                                              at android.app.Dialog$ListenersHandler.handleMessage(Dialog.java:1292)
                                                                              at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                              at android.os.Looper.loop(Looper.java:135)
                                                                              at android.app.ActivityThread.main(ActivityThread.java:5349)
                                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                                              at java.lang.reflect.Method.invoke(Method.java:372)
                                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908)
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)

The question is why?

Answer 1

Make one public class with this method. In this class constructor add Activity object and use this class object to use this method.

    public class TextViewText {

        Activity mActivity;

        public TextViewText (Activity mActivity) {

             this.mActivity = mActivity;
        }

        public void setTextView(String[] countryArray, String column){
        TextView myAwesomeTextView;
        String textViewID;

        for(int i=0; i<=25; i++) {
            if(column.equals("left")){
                textViewID = "textViewColumn1_" + i;
            } else {
                textViewID = "textViewColumn2_" + i;
            }
            int resID = getResources().getIdentifier(textViewID, "id", getPackageName());
            myAwesomeTextView = (TextView) findViewById(resID);
            myAwesomeTextView.setText(countryArray[i]);
        }
    }

}

When you want to set the text :

TextViewText tvt = new TextViewText(MainActivity.this); //your activity context
tvt.setTextView(usa, "left");