CenterOnScreenTwo.java

package javafx;

import javafx.collections.ObservableList;
import javafx.event.EventHandler;
import javafx.geometry.Rectangle2D;
import javafx.stage.Screen;
import javafx.stage.Stage;
import javafx.stage.WindowEvent;

public class CenterOnScreenTwo implements EventHandler<WindowEvent> {
    @Override
    public void handle(WindowEvent windowEvent) {
        Rectangle2D bounds = screen().getVisualBounds();
        double x = bounds.getMinX() + (bounds.getMaxX() - bounds.getMinX()) / 2;
        double y = bounds.getMinY() + (bounds.getMaxY() - bounds.getMinY()) / 2;
        Stage stage = (Stage)windowEvent.getSource();
        stage.setX(x);
        stage.setY(y);
    }

    private Screen screen() {
        ObservableList<Screen> screens = Screen.getScreens();
        Screen screen = screens.get(0);
        if(screens.size()>1) {
            screen = screens.get(1);
        }
        return screen;
    }
}