code: private String[] list(File file) {
          String[] files = (String[]) fileListMap.get(file);
          if (files == null) {
              files = file.list();
              if (files != null) {
                  fileListMap.put(file, files);
              }
          }
          return files;
      }
docs: Return a cached result of list performed on file, if
  available.  Invokes the method and caches the result otherwise.
  
  @param file File (dir) to list.
  @since Ant 1.6