code: public static File absolutize(File base, String path) {
          if (isAbsolute(path))
              return new File(path);
          return new File(base, path);
      }
docs: Resolves the given path with respect to given base. If the path represents an absolute path, a file representing
  it is returned, otherwise a file representing a path relative to base is returned.
  <p>
  It would be nice if File#File(File, String) were doing this.
  @param base File that represents the parent, may be null if path is absolute
  @param path Path of the file, may not be null
  @return new File(name) if name represents an absolute path, new File(base, name) otherwise
  @see hudson.FilePath#absolutize()