What is it? I can't tell you for sure. When I tried to find "exactly" what it was earlier in the day, I didn't find much. All I found in regards to Avast and "Java:Agent-F", was that it was added to Avast's definitions yesterday. Did Avast indicate where it was located?
If it were me, and my A/V detected it, the first thing I would have done was clear Java's cache. It's what I would suggest you do. There's NO harm in doing it.
Go to the Control Panel, and double-click on the Java Icon. On the General tab, under Temporary Internet Files, click the "Settings" button. Leave the "Applications and Applets" and "Trace and Log Files" boxes checked, and then Click OK.
If you have CCleaner installed, you might want to run it for "good measure".