To use JavaShow, you first need to load J/Link using Needs["JLink`"].
window must be an object of a class that can represent a top-level window (i.e., an instance of the java.awt.Window class or a subclass).
Use this function in place of standard Java window methods like setVisible (or its deprecated equivalent, show) and toFront. The effects those methods have are unfortunately not identical on different Java virtual machines and operating systems.
JavaShow performs the required steps in VM-specific ways, so programs that use it will work identically on different configurations.