code: public void addModuleDir(final String moduleDir) {
          if (moduleDir == null) {
              throw LauncherMessages.MESSAGES.nullParam("moduleDir");
          }
          // Validate the path
          final Path path = Paths.get(moduleDir).normalize();
          modulesDirs.add(path.toString());
      }
docs: Adds a directory to the collection of module paths.
  
  @param moduleDir the module directory to add
  
  @throws java.lang.IllegalArgumentException if the path is {@code null}