public void addHeader(@NonNull View view){
  if (view == null) {
    throw new IllegalArgumentException("You can't have a null header!");
  }
  mHeaders.add(view);
}

public void addHeader ( @ NonNull View view ) { if ( view == null ) { throw new IllegalArgumentException ( "You can't have a null header!" ) ; } mHeaders . add ( view ) ; }
adds a header view .
