How can I build an internet download manager in Java?

I am building an Internet Download Manager using java.

How do I get the file size from server? Also, is there a way to divide the file into 开发者_StackOverflow社区partitions so that I can download them in parallel?

First part of your Question:

http://download.oracle.com/javase/1.4.2/docs/api/java/net/URLConnection.html

URL.openConnection().getContentLength()