I made an an attempt to implement OAuth for my programming idea in Java, but I failed miserably. I don't know why, but my code doesn't work. Every time I run my program, an IOException is thrown with the reason "java.io.IOException: Server returned HTTP response code: 401" (401 means Unauthorized). I had a close look at the docs, but I really don't understand why it doesn't work. My OAuth provider I wanted to use is twitter, where I've registered my app, too.
Thanks in advance
phineas

OAuth docs
Twitter API wiki
Class Base64Coder

import java.io.InputStreamReader;
import java.io.BufferedReader; 
import java.io.OutputStream;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.net.URL;
import java.net.URLEncoder;
import java.net.URLConnection;
import java.net.MalformedURLException;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.security.NoSuchAlgorithmException;
import java.security.InvalidKeyException;

public class Request {
    public static String read(String url) {
        StringBuffer buffer = new StringBuffer();
        try {
        /** 
         * get the time - note: value below zero 
         * the millisecond value is used for oauth_nonce later on
         */
            int millis = (int) System.currentTimeMillis() * -1;
            int time = (int) millis / 1000;

            /**
             * Listing of all parameters necessary to retrieve a token
             * (sorted lexicographically as demanded)
             */
             String[][] data = {
                {"oauth_callback", "SOME_URL"},
                {"oauth_consumer_key", "MY_CONSUMER_KEY"},
                {"oauth_nonce",  String.valueOf(millis)},
                {"oauth_signature", ""},
                {"oauth_signature_method", "HMAC-SHA1"},
                {"oauth_timestamp", String.valueOf(time)},
                {"oauth_version", "1.0"}
            };

            /**
             * Generation of the signature base string
             */
            String signature_base_string = 
                "POST&"+URLEncoder.encode(url, "UTF-8")+"&";
            for(int i = 0; i < data.length; i++) {
                // ignore the empty oauth_signature field
                if(i != 3) {
                signature_base_string +=
                    URLEncoder.encode(data[i][0], "UTF-8") + "%3D" +
                    URLEncoder.encode(data[i][1], "UTF-8") + "%26";
                }
            }
            // cut the last appended %26 
            signature_base_string = signature_base_string.substring(0,
                signature_base_string.length()-3);

            /**
             * Sign the request
             */
            Mac m = Mac.getInstance("HmacSHA1");
            m.init(new SecretKeySpec("CONSUMER_SECRET".getBytes(), "HmacSHA1"));
            m.update(signature_base_string.getBytes());
            byte[] res = m.doFinal();
            String sig = String.valueOf(Base64Coder.encode(res));
            data[3][1] = sig;

           /**
            * Create the header for the request
            */
           String header = "OAuth ";
           for(String[] item : data) {
                header += item[0]+"=\""+item[1]+"\", ";
           }
           // cut off last appended comma
           header = header.substring(0, header.length()-2);

           System.out.println("Signature Base String: "+signature_base_string);
           System.out.println("Authorization Header: "+header);
           System.out.println("Signature: "+sig);

           String charset = "UTF-8";
           URLConnection connection = new URL(url).openConnection();
           connection.setDoInput(true);
           connection.setDoOutput(true);
           connection.setRequestProperty("Accept-Charset", charset);
           connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=" + charset);
           connection.setRequestProperty("Authorization", header);
           connection.setRequestProperty("User-Agent", "XXXX");
           OutputStream output = connection.getOutputStream();
           output.write(header.getBytes(charset));

           BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));

           String read;
           while((read = reader.readLine()) != null) {
               buffer.append(read);
           }
        }
        catch(Exception e) {
            e.printStackTrace();
        }

       return buffer.toString();
   }

   public static void main(String[] args) {
       System.out.println(Request.read("http://api.twitter.com/oauth/request_token"));
   }
}