Converting Relative Paths to Absolute Paths

String absolutePath = FileSystems.getDefault().getPath(mayBeRelativePath).normalize().toAbsolutePath().toString();

If I get your problem right, you could do something like this:

File a = new File("/some/abs/path");
File parentFolder = new File(a.getParent());
File b = new File(parentFolder, "../some/relative/path");
String absolute = b.getCanonicalPath(); // may throw IOException