Two classes in Swing could implement java.io interfaces, allowing them to be used in both classes.

javax.swing.filechooser.FileFilter should implement the java.io.FileFilter interface.

javax.swing.filechooser.FileNameExtensionFilter should implement java.io.FilenameFilter. Note that FileNameExtensionFilter only has accept(File). Making it implement the interface will require an accept(File, String), which is simple to add.