Android switchpreference how can i set the switch preference default value?

In your MainActivity onCreate method add this line

PreferenceManager.setDefaultValues(this, R.xml.settings, false);  

You can read about it at http://developer.android.com/reference/android/preference/PreferenceManager.html#setDefaultValues(android.content.Context, int, boolean)

The code is correct just use switchPreferenceCompact instead of SwitchPreference

     <SwitchPreferenceCompat
        app:key="promoNofitficationOnOff"
        app:summaryOff="@string/notification_summary_off"
        app:summaryOn="@string/notification_summary_on"
        app:defaultValue="true"
        app:title="@string/promo_notification_title" />