How did We Get the size of Stage before it was shown??

I set the height and the width of the stage before the start function.

Public void start( Stage primaryStage){

primaryStage.setHeight(500);

primaryStage.setWigth(500);

...

System.out.println(primaryStage.getHeight() +" - "+ primaryStage.getWidth());

...

primaryStage.setScene(scene);

primaryStage.show();

}

If you write "primaryStage.getHeight()" after "primaryStage.setHeight(500);"

the result will be "NaN"

Thank you, hope I help you