Question

I have 2 displays connected, so I can either launch my Java application on the primary or the secondary display.

The question is: How can I know which display contains my app window, i.e., is there a way to detect the current display with Java?

Answer 1

java.awt.Window is the base class of all top level windows (Frame, JFrame, Dialog, etc.) and it contains the getGraphicsConfiguration() method that returns the GraphicsConfiguration that window is using. GraphicsConfiguration has the getGraphicsDevice() method which returns the GraphicsDevice that the GraphicsConfiguration belongs to. You can then use the GraphicsEnvironment class to test this against all GraphicsDevices in the system, and see which one the Window belongs to.

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);