Can I create Webpart in Java and deploy it to SharePoint 2010? What basics needs?
In addition to what Steve said, you can't create a webpart using Java language for SharePoint framework and I don't think there should be any need for it either, its more like you want to play games developed for "Play Station 3" on "Xbox-360" anyway I find out that there are few ways you can show java portlets in SharePoint webparts using Web Services for Remote Portlets if that helps.
You can read more about it on this Microsoft's blog or simply download WSRP toolkit released by Microsoft.
I also find few useful links that might help you for what you are trying to achieve,
Oh and one thing I forgot to mention and that is Webpart is a ASP.NET control and is also rendered by asp.net (dunno if you know or not but sharepoint is developed on top of ASP.Net) so you can't develop it in Java as you won't going to have access to parent classes if I make any sense.
Hope it helps, but if you have some specific requirements let us know and we will try to help you accordingly as much as we could.
No you can't, SharePoint is based on .Net and then requires you to build .Net apps (C#, VB.net or any cls compliant language)