Currently, java.awt.Window(GraphicsConfiguration gc) costructor throws IllegalArgumentException in headless mode. That causes all subclasses (java.awt.Frame, java.awt.Dialog, javax.swing.JWindow, and others) to throw IAE also. It seems strange to throw this exception, as a separate exception HeadlessException exists for this purpose.

This is an incompatible change as neither IAE nor HeadlessException are subclasses of each other. However, both of them are runtime (unchecked) exceptions, and impact of this change is very low.