In most operating systems, there's a "default application" for a certain type of file
Not really, this mapping from file types to application is usually either application or graphic environment dependant. Only windows is an exception to that.
Java is not a graphic environment (like Gnome, KDE and the likes are), so hasn't this feature built-in.
However Java can integrate deeply into some of the graphic environments it can run in with JDIC (https://jdic.dev.java.net/
), which is really what you are looking for I guess.
In particular, have a look at the classes org.jdesktop.jdic.desktop.Desktop and org.jdesktop.jdic.filetypes.Association.