Implementation of Hhhash in Java (HTTP Header Hashing)

Hhhash.java

import          java.io.IOException;
import          java.net.MalformedURLException;
import          java.net.URL;
import          java.net.URLConnection;
import          java.util.ArrayList;
import          java.util.List;
import          java.util.Map;
import          java.security.MessageDigest;

/*
Implementation of Hhhash in Java
HTTP Header Hashing
Used to group hosts together ... Take the keys in the order presented by the server 
and concat them with ":" and SHA-256 hash that

https://www.foo.be/2023/07/HTTP-Headers-Hashing_HHHash

NOTE
if some of this looks wonky it's because i developed it on a Java 1.6 system, and 
since them some things i had to do are now in the standard library (like String.join). 
 */

public class    Hhhash {
        private static String join(String separator, List < String > input) {
                if (input == null || input.size() <= 0)
                        return "";

                StringBuilder   sb = new StringBuilder();

                for             (int i = 0; i < input.size(); i++) {
                        sb.append(input.get(i));
                        //if not the last item
                        if (i != input.size() - 1) {
                                sb.append(separator);
                        }
                }
                return sb.toString();

        }

        public static String getSha256(String value) {
                try {
                        MessageDigest   md = MessageDigest.getInstance("SHA-256");
                        md.update(value.getBytes());
                        return bytesToHex(md.digest());
                } catch(Exception ex) {
                        throw new       RuntimeException(ex);
                }
        }

        private static String bytesToHex(byte[] bytes) {
                StringBuffer    result = new StringBuffer();
                for       (byte b:                bytes)
                           result.append(Integer.toString((b & 0xff) + 0x100, 16).substring(1));
                return result.toString();
        }

        public static void main(String[] args) {
                for (String s:args) {
                        List<String> l = new ArrayList < String > ();
                        try {
                                URL             u = new URL(s);
                                URLConnection   conn = u.openConnection();
                                Map<String, List<String>> map = conn.getHeaderFields();
                                for  (Map.Entry<String, List<String>> entry:                map.entrySet()) {
                                        l.add(entry.getKey());
                                }
                                String hhhval = getSha256(join(":", l));
                                System.out.println(u.getHost() + " -> hhh:1:" + hhhval);
                        }  catch(IOException e) {
                                System.out.println(e.getMessage());
                                continue;
                        }
                }
        }
}