code: public void warn( Element e, String msg, Object... args ) {
          messager.printMessage( Diagnostic.Kind.WARNING, String.format( msg, args ), e );
      }
docs: Prints a warning message
  
  @param e The element which has caused the error. Can be null
  @param msg The error message
  @param args if the error message contains %s, %d etc. placeholders this arguments will be used
  to replace them