The spec of GraphicsDevice.setFullScreenWindow is silent about the method behavior when other window is in full-screen mode already. The spec states: "Windows cannot overlap the full-screen window." However calling this method for other (non full-screen) window returns current full-screen window to windowed mode and enters the second one to full-screen instead. This behavior is not described in the spec.

Code below illustrates current behavior: it shows initially blue full-screen window and then yellow one (blue is returned to windowd mode):
=================== Test44.java ===========================
import java.awt.*;

public class Test44 {

    public static void main(String[] argv) {
        Test44 t = new Test44();
        t.go();
    }
    public void go() {
        GraphicsDevice gd = GraphicsEnvironment.
            getLocalGraphicsEnvironment().
            getDefaultScreenDevice();
    
        Window w1 = new Window(new Frame("one"));
        w1.setSize(200, 200);
        w1.setBackground(Color.blue);
        gd.setFullScreenWindow(w1);
        try {
            Thread.sleep(3000);
        } catch (InterruptedException ie) {
        }

    
        Window w2 = new Window(new Frame("two"));
        w2.setBackground(Color.yellow);
        gd.setFullScreenWindow(w2);

        try {
            Thread.sleep(3000);
        } catch (InterruptedException ie) {
        }
        System.exit(0);
    }
}
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###@###.### 2004-12-10 13:44:56 GMT