Generating unique strings with JAVA

If you need randomly generated Strings in your java code you can use the below functions.

01  public String generateRandomString(String s) { 02   try { 03    SecureRandom prng = SecureRandom.getInstance("SHA1PRNG"); 04    String randomNum = new Integer(prng.nextInt()).toString(); 05    randomNum += s; 06    MessageDigest sha = MessageDigest.getInstance("SHA-1"); 07    byte[] result = sha.digest(randomNum.getBytes()); 08    return hexEncode(result); 09   } catch (NoSuchAlgorithmException e) { 10    return  System.currentTimeMillis()+"_"+username; 11   } 12  }

The classical hexEncode method:

01 protected String hexEncode(byte[] aInput) { 02   StringBuffer result = new StringBuffer(); 03   char[] digits = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 04     'a', 'b', 'c', 'd', 'e', 'f' }; 05   for (int idx = 0; idx < aInput.length; ++idx) { 06    byte b = aInput[idx]; 07    result.append(digits[(b & 0xf0) >> 4]); 08    result.append(digits[b & 0x0f]); 09   } 10   return result.toString(); 11  }

Suppose that you need to generate unique random session ids for your logged in users. You can use the above function as follows :

       String sessionId = generateRandomString(username);

10 consecutive calls to generateRandomString("ilkinulas") generates the following strings:

38aca43c835888bc38fbb2d431f537489d637427
8c9fec7eb1309cd60a9548a11b16a30c2cf277a4
d495e83405704d38bfdf1bc3e2a38b31a5b52243
e3f15b900a3b80936596dc4471c0e42889dbf00c
83a4c869593af633a97b3b68433fac19bac7937d
f7573e8fdb4ecf6f58062570b66943b08d87fab8
c19bfdf75abbafbb5d9ca0ce37b90e97f08fccfa
41f0d9cd01ff814f8879bbff4b06642fc01a4624
aae1a792389239ca8db35a1ff9b16856c9921845
c2c9041ab5039b38c79a125c57a10cc6a3eabad9