Overview
In this tutorial, we show you how to a space between Buttons in HBox JavaFX.
Adding space between buttons in HBox by add Region()
We add a 
Region with 
Hgrow set to ALWAYS between the Buttons.
        Region spacer = new Region();
        HBox.setHgrow(spacer, Priority.ALWAYS);
Example add a Region between OK Button and Cancel Button
    @FXML
    private HBox hbxButtons;
    /**
     * Initializes the controller class.
     */
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        Button btnOk = new Button();
        btnOk.setText("OK");
        btnOk.setPrefWidth(100);
        btnOk.setPrefHeight(30);
        hbxButtons.getChildren().add(btnOk);
        Region spacer = new Region();
        HBox.setHgrow(spacer, Priority.ALWAYS);
        hbxButtons.getChildren().add(spacer);
        Button btnCancel = new Button();
        btnCancel.setText("Cancel");
        btnCancel.setPrefWidth(100);
        btnCancel.setPrefHeight(30);
        hbxButtons.getChildren().add(btnCancel);
        
    }
Adding space between buttons in HBox by call setSpacing method
Call 
setSpacing method of HBox
package com.jackrutorial;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
public class ExampleController implements Initializable {
    @FXML
    private HBox hbxButtons;
    /**
     * Initializes the controller class.
     */
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        Button btnOk = new Button();
        btnOk.setText("OK");
        btnOk.setPrefWidth(100);
        btnOk.setPrefHeight(30);
        hbxButtons.getChildren().add(btnOk);
        Button btnCancel = new Button();
        btnCancel.setText("Cancel");
        btnCancel.setPrefWidth(100);
        btnCancel.setPrefHeight(30);
        hbxButtons.getChildren().add(btnCancel);
        
        hbxButtons.setSpacing(20);
        
    }
}
Adding space between buttons in HBox by edit FXML file
Edit FXML file, then add the spacing attribute 
spacing="20"
 <HBox fx:id="hbxButtons" layoutX="150.0" layoutY="150.0" prefHeight="50.0" prefWidth="300.0" spacing="20" />