InputStream from a URL

How do I get an InputStream from a URL?

for example, I want to take the file at the url wwww.somewebsite.com/a.txt and read it as an InputStream in Java, through a servlet.

I've tried

InputStream is = new FileInputStream("wwww.somewebsite.com/a.txt");

but what I got was an error:

java.io.FileNotFoundException