code: public static String getPathsString(Collection<VirtualFile> paths) {
          if (paths == null) {
              throw MESSAGES.nullArgument("paths");
          }
          StringBuilder buffer = new StringBuilder();
          boolean first = true;
          for (VirtualFile path : paths) {
              if (path == null) { throw new IllegalArgumentException("Null path in " + paths); }
              if (first == false) {
                  buffer.append(':');
              } else {
                  first = false;
              }
              buffer.append(path.getPathName());
          }
          if (first == true) {
              buffer.append("<empty>");
          }
          return buffer.toString();
      }
docs: Get the paths string for a collection of virtual files
  
  @param paths the paths
  @return the string
  @throws IllegalArgumentException for null paths