code: public static File subDir(final String parent, final String child) {
  		final File d = new File(child);
  		return d.isAbsolute() ? d : new File(parent, child);
  	}
docs: Returns a {@link File} object representing a child relative
  to a parent, or just the child, if absolute.
  
  @param parent a parent directory.
  @param child a (possibly absolute) child file or directory.
  @return {@code child}, if it is absolute; {@code child} relative to {@code parent}, if it is relative.