node's coordinates in javafx - javafx

Question

I have a question.. well, I use this code to get my node's coordinates
Bounds localToScreen = myNode.localToScene(myNode.getBoundsInLocal());
localToScreen.getMinX();
localToScreen.getMinY();
localToScreen.getWidth();
localToScreen.getHeight();
but this code just sometimes works and In the other times it shows wrong results.. so is there any thing special about using this code? why doesn't it work any times? In scenebuilder all the node's coordinates are right.
this is an example
my fxml file is
<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<AnchorPane fx:id="anchor" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="test.FXMLDocumentController">
<children>
<HBox fx:id="hbox" layoutX="147.0" layoutY="67.0" prefHeight="34.0" prefWidth="200.0">
<children>
<TextField />
<Button mnemonicParsing="false" text="Button" />
</children>
</HBox>
</children>
</AnchorPane>
and my controller is
public class FXMLDocumentController implements Initializable {
#FXML
private AnchorPane anchor;
#FXML
private HBox hbox;
/**
* Initializes the controller class.
*/
#Override
public void initialize(URL url, ResourceBundle rb) {
Bounds localToScreen = hbox.localToScene(hbox.getBoundsInLocal());
System.out.print(""+
localToScreen.getMinX() +" "+
localToScreen.getMinY() +" "+
localToScreen.getWidth()+" "+
localToScreen.getHeight()
);
// TODO
}
the result is
147.0 67.0 0.0 0.0
width and height should not be 0!
please help.. thanks