how?
Check the javadocs for:
java.awt.GraphicsEnvironment
java.awt.GraphicsDevice.setFullScreenWindow(Window)
java.awt.GraphicsDevice.setDisplayMode(DisplayMode)
java.awt.DisplayMode
you should read the java-tutorial about it here:
http://java.sun.com/docs/books/tutorial/extra/fullscreen/index.html
Some of the nehe tutorials support fullscreen.