Koronaa

public static String getPseudoUniqueID() {
    /*
    Initially we append all the information with the number "35" so all together we get 
    17 characters and the generatedId starts from 35 which makes it look like a IMEI. 

    But later I commented out DISPLAY,HOST and ID because they can be changed and 
    then the generated ID won't be unique anymore
    */
    String generatedID = "35" + Build.BOARD.length() % 10 + Build.BRAND.length() % 10 +
        Build.CPU_ABI.length() % 10 + Build.DEVICE.length() % 10 +

Koronaa

String uniqueID = UUID.randomUUID().toString();

Koronaa

//Once you handle all the runtime permission drama for android.permission.READ_PHONE_STATE
TelephonyManager telephonyManager = (TelephonyManager)getSystemService(Context.TELEPHONY_SERVICE);
telephonyManager.getDeviceId();

Koronaa

String secureId = Settings.Secure.getString(getContentResolver(), Settings.Secure.ANDROID_ID);