why checkboxpreference is a null object preference

Question

I have problem where received null object reference at uncheckAll() method inside HomeActivity class. But the problem is I already reference both object ( checkboxPreferenceEvent and checkBoxPreferenceQuote ) inside MyPreferenceActivity class. Hope you guys can help me solve this problem.

Thanks..

checkBoxPreferenceEvent = checkBoxPreferenceArrayList.get(0);
checkBoxPreferenceQuote = checkBoxPreferenceArrayList.get(1);

ERROR

E/InputEventReceiver: Exception dispatching input event.
E/MessageQueue-JNI: Exception in MessageQueue callback: handleReceiveCallback
E/MessageQueue-JNI: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.preference.CheckBoxPreference.setChecked(boolean)' on a null object reference
                        at com.example.user.uniselic.MyPreferencesActivity.uncheckAll(MyPreferencesActivity.java:51)
                        at com.example.user.uniselic.HomeActivity.onOptionsItemSelected(HomeActivity.java:193)
                        at android.app.Activity.onMenuItemSelected(Activity.java:2928)
                        at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:406)
                        at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:195)
                        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:103)

MyPreferencesActivity.java

package com.example.user.uniselic;

import android.content.Context;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.preference.CheckBoxPreference;
import android.preference.Preference;
import android.preference.PreferenceActivity;
import android.preference.PreferenceFragment;
import android.preference.PreferenceScreen;
import android.preference.TwoStatePreference;
import android.util.Log;
import android.widget.Toast;

import com.google.firebase.messaging.FirebaseMessaging;

import java.util.ArrayList;


/**
 * Created by User on 11/1/2016.
 */

public class MyPreferencesActivity extends PreferenceActivity{

    ArrayList<CheckBoxPreference> checkBoxPreferenceArrayList;
    CheckBoxPreference checkBoxPreferenceQuote;
    CheckBoxPreference checkBoxPreferenceEvent;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        MyPreferenceFragment myPreferenceFragment = new MyPreferenceFragment();
        getFragmentManager().beginTransaction().replace(android.R.id.content,myPreferenceFragment).commit();

        checkBoxPreferenceArrayList = myPreferenceFragment.getCheckBoxPreferenceArrayListInner();

        if(!checkBoxPreferenceArrayList.isEmpty()){
            checkBoxPreferenceEvent = checkBoxPreferenceArrayList.get(0);
            checkBoxPreferenceQuote = checkBoxPreferenceArrayList.get(1);
            Toast.makeText(MyPreferencesActivity.this,"size arraylist:"+checkBoxPreferenceArrayList.size(),Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(MyPreferencesActivity.this,"listview is empty",Toast.LENGTH_SHORT).show();
        }

    }

    public void uncheckAll(){
        Log.d("testing","calling uncheckAll method");
        checkBoxPreferenceEvent.setChecked(false);
        checkBoxPreferenceQuote.setChecked(false);


    }

    public class  MyPreferenceFragment extends PreferenceFragment{

        CheckBoxPreference checkBoxPreferenceEvent;
        CheckBoxPreference checkBoxPreferenceQuote;
        ArrayList<CheckBoxPreference> checkBoxPreferenceArrayListInner = new ArrayList<>();

        public MyPreferenceFragment(){

            checkBoxPreferenceArrayListInner.add(checkBoxPreferenceEvent);
            checkBoxPreferenceArrayListInner.add(checkBoxPreferenceQuote);

        }
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);

            addPreferencesFromResource(R.xml.preferences);

            //initialize checkbox preferences.
            checkBoxPreferenceEvent = (CheckBoxPreference) findPreference("applicationUpdatesEvent");
            checkBoxPreferenceQuote = (CheckBoxPreference) findPreference("applicationUpdatesQuote");


            //listener for checkbox event
            checkBoxPreferenceEvent.setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
                @Override
                public boolean onPreferenceClick(Preference preference) {
                    PreferenceScreen preferenceScreen = getPreferenceScreen();
                    if(((CheckBoxPreference)preferenceScreen.findPreference("applicationUpdatesEvent")).isChecked()){

                        //initalize sharepreferences
                        SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences(ConfigBundle.SETTING, Context.MODE_PRIVATE);
                        //set value on shareprefenrence.
                        SharedPreferences.Editor editor = sharedPreferences.edit();
                        editor.putBoolean(ConfigBundle.SETTING_NOTIFICATION_EVENT,true);
                        editor.commit();
                        FirebaseMessaging.getInstance().subscribeToTopic("event");
                        Toast.makeText(getActivity(),"Event Notification Enable",Toast.LENGTH_LONG).show();
                        return true;
                    }else{
                        //initalize sharepreferences
                        SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences(ConfigBundle.SETTING, Context.MODE_PRIVATE);
                        //set value on shareprefenrence.
                        SharedPreferences.Editor editor = sharedPreferences.edit();
                        editor.putBoolean(ConfigBundle.SETTING_NOTIFICATION_EVENT,false);
                        editor.commit();
                        FirebaseMessaging.getInstance().unsubscribeFromTopic("event");
                        Toast.makeText(getActivity(),"Event Notification Disable",Toast.LENGTH_LONG).show();
                        return false;
                    }
                }
            });

            //listener for checkbox quote
            checkBoxPreferenceQuote.setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
                @Override
                public boolean onPreferenceClick(Preference preference) {
                    PreferenceScreen preferenceScreen = getPreferenceScreen();
                    if(((CheckBoxPreference)preferenceScreen.findPreference("applicationUpdatesQuote")).isChecked()){

                        //initalize sharepreferences
                        SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences(ConfigBundle.SETTING, Context.MODE_PRIVATE);
                        //set value on shareprefenrence.
                        SharedPreferences.Editor editor = sharedPreferences.edit();
                        editor.putBoolean(ConfigBundle.SETTING_NOTIFICATION_QUOTE,true);
                        editor.commit();
                        FirebaseMessaging.getInstance().subscribeToTopic("quote");
                        Toast.makeText(getActivity(),"Quote Notification Enable",Toast.LENGTH_SHORT).show();
                        return true;
                    }else{
                        //initalize sharepreferences
                        SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences(ConfigBundle.SETTING, Context.MODE_PRIVATE);
                        //set value on shareprefenrence.
                        SharedPreferences.Editor editor = sharedPreferences.edit();
                        editor.putBoolean(ConfigBundle.SETTING_NOTIFICATION_QUOTE,false);
                        editor.commit();
                        FirebaseMessaging.getInstance().unsubscribeFromTopic("quote");
                        Toast.makeText(getActivity(),"Quote Notification Disable",Toast.LENGTH_SHORT).show();
                        return false;
                    }
                }
            });


        }

        //get arraylist that contain checkboxpreference
        public ArrayList<CheckBoxPreference> getCheckBoxPreferenceArrayListInner(){
            return checkBoxPreferenceArrayListInner;
        }


    }



}

HomeActivity.java

/*To make our option menu active,ovveride this method
    so that we can add function fo each option menu that been created */
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()){
            case R.id.menu_1:
                Intent intent = new Intent(HomeActivity.this,ContactActivity.class);
                startActivity(intent);
                break;
            case R.id.menu_2:
                Intent intent2 = new Intent(HomeActivity.this,MyPreferencesActivity.class);
                startActivity(intent2);
                break;
            case R.id.menu_3:
                //remove all information user from sharepreference.
                SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences(ConfigBundle.SETTING, Context.MODE_PRIVATE);
                SharedPreferences.Editor editor = sharedPreferences.edit();
                editor.putBoolean(ConfigBundle.SETTING_FIRST_TIME,true);
                editor.putString(ConfigBundle.SETTING_ID,"");
                editor.putString(ConfigBundle.SETTING_EMAIL,"");
                editor.putString(ConfigBundle.SETTING_USERNAME,"");
                editor.putBoolean(ConfigBundle.SETTING_NOTIFICATION_EVENT,false);
                editor.putBoolean(ConfigBundle.SETTING_NOTIFICATION_QUOTE,false);
                editor.commit();

                MyPreferencesActivity myPreferencesActivity = new MyPreferencesActivity();
                myPreferencesActivity.uncheckAll();

                FirebaseMessaging.getInstance().unsubscribeFromTopic("event");
                FirebaseMessaging.getInstance().unsubscribeFromTopic("quote");
                FirebaseMessaging.getInstance().unsubscribeFromTopic("news");

                //then go to main login activity content.
                Intent intent3 = new Intent(HomeActivity.this,LoginActivity.class);
                startActivity(intent3);
        }
        return super.onOptionsItemSelected(item);
    }

Answer 1

Seems that your Activity does not have a ContentView to init.:

setContentView(R.layout.your_view);