Sometimes we need to generate a unique number for an emailId, for example when a user register then we need to generate a number by using that user can verify his EmailId. For this we can use SHA1 encoding method.

Example - In the below method I'm generating unique number for the provided emailId by using SHA1.

    import java.security.MessageDigest;
    import java.security.NoSuchAlgorithmException;

    /**
     * This method will generate unique code based on emailID
     * @param emailID
     * @return
     */
    public static String generateEncodedKey(String emailID)
    {
         String generatedKey="";
     try {
            MessageDigest md = MessageDigest.getInstance("SHA1");
            md.update(emailID.getBytes());
            byte[] bytes = md.digest();
            StringBuilder sb = new StringBuilder();
            for(int i=0; i< bytes.length ;i++){
                sb.append(Integer.toString((bytes[i] & 0xff) + 0x100, 16).substring(1));
            }
            generatedKey = sb.toString().substring(0, 8);
        }
        catch (NoSuchAlgorithmException e){
            e.printStackTrace();
        }

        return generatedKey;
    }

Hope this will help you :)