JAVA

java active displays

Window myWindow = ....
// ...
GraphicsConfiguration config = myWindow.getGraphicsConfiguration();
GraphicsDevice myScreen = config.getDevice();
GraphicsEnvironment env = GraphicsEnvironment.getLocalGraphicsEnvironment();
// AFAIK - there are no guarantees that screen devices are in order... 
// but they have been on every system I've used.
GraphicsDevice[] allScreens = env.getScreenDevices();
int myScreenIndex = -1;
for (int i = 0; i < allScreens.length; i++) {
    if (allScreens[i].equals(myScreen))
    {
        myScreenIndex = i;
        break;
    }
}
System.out.println("window is on screen" + myScreenIndex);

java active displays

    public static GraphicsDevice getWindowDevice(Window window) {
    Rectangle bounds = window.getBounds();
    return asList(GraphicsEnvironment.getLocalGraphicsEnvironment().getScreenDevices()).stream()

            // pick devices where window located
            .filter(d -> d.getDefaultConfiguration().getBounds().intersects(bounds))

            // sort by biggest intersection square
            .sorted((f, s) -> Long.compare(//
                    square(f.getDefaultConfiguration().getBounds().intersection(bounds)),
                    square(s.getDefaultConfiguration().getBounds().intersection(bounds))))

            // use one with the biggest part of the window
            .reduce((f, s) -> s) //

            // fallback to default device
            .orElse(window.getGraphicsConfiguration().getDevice());
}

public static long square(Rectangle rec) {
    return Math.abs(rec.width * rec.height);
}