import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class FullScreenTest extends Application {
    public static void main(String[] args) {
        launch(args);
    }

	@Override
	public void start(Stage primaryStage) throws Exception {
		primaryStage.setTitle("Hello World!");
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnAction(new EventHandler<ActionEvent>() {
 
            @Override
            public void handle(ActionEvent event) {
                System.out.println("Hello World!");
            }
        });
        
        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
        
        primaryStage.setFullScreen(true); 

        Alert alert = new Alert(Alert.AlertType.ERROR); 
        alert.setTitle("Alert"); 
        alert.setContentText("Alert"); 
        alert.initOwner(primaryStage); 

        alert.showAndWait(); 
		
	}
}