Java - SimpleDateFormat formatter to return epoch time with milliseconds

I am very new to Java and coding in general - I have some code which returns a timestamp in the following format yyyy.MM.dd HH:mm:ss:ms which is shown below:

SimpleDateFormat formatter = new SimpleDateFormat("yyyy.MM.dd HH:mm:ss:sss");

This returns:

2017.07.19 11:42:30:423

Is there a way to edit the "SimpleDateFormat formatter" code above to return the date/time as an epoch timestamp that includes milliseconds so that the value returned is formatted as per the below?

1500464550423

I'm hoping that I can amend the ("yyyy.MM.dd HH:mm:ss:sss") part of the SimpleDateFormat formatter code to do this.

Any help or advice is much appreciated.

Thanks