Currently JFrame and JInternalFrame do not have similar window management
strategies. e.g. you can register a WindowListener on JFrame,
whereas you have to subclass JInternalFrame and override methods like setSelected(), setIncon() to implement similar functionality.

Methods like pack() are missing in JInternalFrame().

This makes programmers job and makes unnecessary duplication of functionality which is really should be common.

These classes should only differ in terms of being a native toplevel frame
and being an internal frame i.e. Window Manager interaction