Three different methods to get unique identifier on Android, they are all not the perfect solutions however could be used under project restriction limit.

DeviceUniqueUtil.java

/**
 * Three different methods to get unique identifier on Android, they are all not the perfect solutions however could be used under project restriction limit.
 * 
 * @author Xinyue Zhao
 */
public class DeviceUniqueUtil {
    /**
     * Android UUID method. It could be an unique solution.
     * See. http://developer.android.com/reference/java/util/UUID.html
     */
    public String getAndroidUUID(Context _c) {
        return UUID.randomUUID().toString();
    }

    /**
     * Get first primary GMAIL account as an unique bundle.
     */
    public String getAndroidPrimaryGmail(Context _c) {
        if (_c != null) {
            Pattern emailPattern = Patterns.EMAIL_ADDRESS;
            Account[] accounts = AccountManager.get(_c).getAccountsByType("com.google");
            for (Account account: accounts) {
                if (emailPattern.matcher(account.name).matches()) {
                    String email = account.name;
                    String manufactor = Build.MANUFACTURER;
                    String model = Build.MODEL;
                    String serial = Build.SERIAL;
                    StringBuilder sb = new StringBuilder();
                    appendNonEmptyString(email, sb);
                    appendNonEmptyString(manufactor, sb);
                    appendNonEmptyString(model, sb);
                    appendNonEmptyString(serial, sb);
                    if (isStandalone()) {
                        sb.append(getCenterId());
                    }

                    try {
                        deviceId = Util.SHA1(sb.toString());
                    } catch (NoSuchAlgorithmException _e) {
                        _e.printStackTrace();
                    } catch (UnsupportedEncodingException _e) {
                        _e.printStackTrace();
                    }
                    break;
                }
            }
        }
        return deviceId;
    }

    /**
     * Fetch a MD5-Hash by getting IMEI or MAC or ANDROID's Secure-ID.
     */
    private static class DeviceId {
        /**
         * requires permissions TELEPHONY_SERVICE on cell phones and WIFI_SERVICE on WiFi-only devices
         *
         * @param Context
         *            _cxt the current activity
         * @return String
         * @throws Exception
         *             when there is no unique ID
         */
        static public String getDeviceId(Context _cxt) throws Exception {
            String readableId;
            // Requires READ_PHONE_STATE
            TelephonyManager tm = (TelephonyManager)
            _cxt.getSystemService(Context.TELEPHONY_SERVICE);
            // gets the imei (GSM) or MEID/ESN (CDMA)
            String imei = tm.getDeviceId();
            if (null != imei && imei.length() > 0 && Long.parseLong(imei) > 0) { // 000000000000000 for emulator
                readableId = imei;
            } else {
                // devices without SIM card (e.g. WiFi-only tablets)
                // requires ACCESS_WIFI_STATE
                WifiManager wm = (WifiManager) _cxt.getSystemService(Context.WIFI_SERVICE);
                // gets the MAC address
                String mac = wm.getConnectionInfo().getMacAddress();
                if (null != mac && mac.length() > 0) {
                    readableId = mac;
                } else {
                    // gets the android-assigned id
                    // unfortunately, this is not unique on some devices:
                    // http://groups.google.com/group/android-developers/browse_thread/thread/53898e508fab44f6/84e54feb28272384?pli=1
                    // so it's only a fallback for emulators
                    String androidId = Secure.getString(_cxt.getContentResolver(), Secure.ANDROID_ID);
                    if (null != androidId && androidId.length() > 0) {
                        readableId = androidId;
                    } else {
                        throw new Exception("cannot calculate a unique device ID");
                    }
                }
            }
            return md5(readableId);
        }

        private static String md5(String _plaintext) throws NoSuchAlgorithmException {
            MessageDigest digest = java.security.MessageDigest.getInstance("MD5");
            digest.update(_plaintext.getBytes());
            byte[] hash = digest.digest();
            BigInteger bigInt = new BigInteger(1, hash);
            return String.format("%1$032X", bigInt).toLowerCase();
        }
    }
}