In Java, what are the performance and resource implications of using
System.currentTimeMillis() is obviously the most efficient since it does not even create an object, but
new Date() is really just a thin wrapper about a long, so it is not far behind.
Calendar, on the other hand, is relatively slow and very complex, since it has to deal with the considerably complexity and all the oddities that are inherent to dates and times (leap years, daylight savings, timezones, etc.).
It's generally a good idea to deal only with long timestamps or
Date objects within your application, and only use
Calendar when you actually need to perform date/time calculations, or to format dates for displaying them to the user. If you have to do a lot of this, using Joda Time is probably a good idea, for the cleaner interface and better performance.