Cursor c = getContentResolver().query(
        RawContacts.CONTENT_URI,new String[] { RawContacts.CONTACT_ID,RawContacts.DISPLAY_NAME_PRIMARY },RawContacts.ACCOUNT_TYPE + "= ?",new String[] { "com.whatsapp" },null);

ArrayList<String> myWhatsappContacts = new ArrayList<String>();
int contactNameColumn = c.getColumnIndex(RawContacts.DISPLAY_NAME_PRIMARY);
while (c.moveToNext())
{
    // You can also read RawContacts.CONTACT_ID to read the
    // ContactsContract.Contacts table or any of the other related ones.
    myWhatsappContacts.add(c.getString(contactNameColumn));
}