How do I use ag to look for text in files with certain extensions?

I'm using ag (The Silver Searcher ) version 0.31.0. I can easily look for a string in a bunch of files using: localhost:workspace davea$ ag 'ftp' . But what if I only want to scan files with certain extensions? I tried this: localhost:workspace davea$ ag 'ftp' .java ERR: Error stat()ing: .java ERR: Error opening directory .java: No such file or directory but got the errors you see above.