How to get rid of the starting slash in URI or URL?

Convert the URL to a URI and use that in the File constructor:

URL res = this.getClass().getClassLoader().getResource(dictionaryPath);
File file = new File(res.toURI());
String fileName = file.getPath();

As long as UNIX paths are not supposed to contain drive letters, you may try this:

URL res = this.getClass().getClassLoader().getResource(dictionaryPath);
String path = res.getPath();
char a_char = text.charAt(2);
if (a_char==':') path = path.substring(1);

Convert to a URI, then use Paths.get().

URL res = this.getClass().getClassLoader().getResource(dictionaryPath);
String path = Paths.get(res.toURI()).toString();