Do you mean that SharePoint can be extended by writing some code? If so, I suspect that such code is written in C# and uses various SharePoint services. I don't see any easy way to convert something like that into Java - the C# code would not be that difficult but replacing the SharePoint services would be very difficult.
Or do you mean how can a web site based on SharePoint be converted into Java? If that is what you mean, then you should look into various Content Management System solutions such as Alfresco or Magnolia CRM. It will not be exactly the same, but close.