support for "<? extends X>" and "<? super X>" is supporting covariant / contravariant. invariant would be "<x>". basically. the same concept applies in a num

support for "<? extends X>" and "<? super X>" is supporting covariant / contravariant. invariant would be "<x>". basically. the same concept applies in a number of areas, with varying support (e.g. Java doesn't support contravariance in its method overloading: http://stackoverflow.com/a/2501513/16295 http://stackoverflow.com/a/2501513/16295 )