code: public static boolean isRelativePath(String path) {
          if (path.startsWith("/"))
              return false;
          if (path.startsWith("\\\\") && path.length() > 3 && path.indexOf('\\', 3) != -1)
              return false; // a UNC path which is the most absolute you can get on windows
          if (path.length() >= 3 && ':' == path.charAt(1)) {
              // never mind that the drive mappings can be changed between sessions, we just want to
              // know if the 3rd character is a `\` (or a '/' is acceptable too)
              char p = path.charAt(0);
              if (('A' <= p && p <= 'Z') || ('a' <= p && p <= 'z')) {
                  return path.charAt(2) != '\\' && path.charAt(2) != '/';
              }
          }
          return true;
      }
docs: A mostly accurate check of whether a path is a relative path or not. This is designed to take a path against
  an unknown operating system so may give invalid results.
  
  @param path the path.
  @return {@code true} if the path looks relative.
  @since 1.606