How do I create a JavaFX BorderPane with no center?

I would like to create a BorderPane layout in JavaFX with no center pane.

The code I have written so far only implements the left and right borders and is below:

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class GUI_Practice extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        String blackBorder = "-fx-border-style: solid; -fx-border-width: 1; -fx-border-color: black";

        /* Left column */
        Button save = new Button("Save");
        Button del = new Button("Delete");
        HBox settings = new HBox(save, del);
        VBox leftCol = new VBox(settings);
        leftCol.setStyle(blackBorder);

        /* Right column */
        Button calculate = new Button("Calculate");
        Button cancel = new Button("Cancel");
        HBox runButtons = new HBox(calculate, cancel);
        VBox rightCol = new VBox(runButtons);
        rightCol.setStyle(blackBorder);

        /* Set up borderpane */
        BorderPane root = new BorderPane();
        root.setPadding(new Insets(15));
        root.setLeft(leftCol);
        root.setRight(rightCol);

        Scene scene = new Scene(root, 800, 600);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch();
    }

}

The output it gives is shown in the image below:

However, I want it to look more like this:

Where the left and right columns are equal width and take up the entire width of the window. Additionally, the columns do not change width with the window, so the whitespace in the middle gets bigger as the window gets bigger.

What do I need to change to make the columns fill the width of the window?

(P.S. I'm still learning, so if the solution could avoid FXML (which I don't understand yet), that'd be great)

EDIT: as per @k88's suggestion, my start method now looks like so:

public void start(Stage stage) throws Exception {
        String blackBorder = "-fx-border-style: solid; -fx-border-width: 1; -fx-border-color: black";

        Button calculate = new Button("Calculate");
        Button cancel = new Button("Cancel");
        HBox runButtons = new HBox(calculate, cancel);
        VBox rightCol = new VBox(runButtons);
        rightCol.setStyle(blackBorder);

        Button save = new Button("Save");
        Button del= new Button("Delete");
        HBox settings = new HBox(save, load);
        VBox leftCol = new VBox(settings);
        leftCol.setStyle(blackBorder);

        HBox root = new HBox(leftCol, rightCol);
        root.setPadding(new Insets(15));

        Scene scene = new Scene(root, 800, 600);
        stage.setScene(scene);
        stage.show();
    }

Giving a window looking like:

There are different ways to get this problem fixed.

1. If you want to still gain the benefits from BorderPane (like to have top and bottom panes), you can set a HBox/GridPane as the center (without setting left/right).
2. If you are not bothered about top and bottom layout implementations, then as @k88 suggested, you can use directly HBox or GridPane as your root node.

Using HBox:

HBox.setHGrow(leftCol,Priority.ALWAYS);
HBox.setHGrow(rightCol,Priority.ALWAYS);
HBox root = new HBox();
root.setPadding(new Insets(15));
root.getChildren().addAll(leftCol, rightCol);

Using GridPane:

GridPane root = new GridPane();
ColumnConstraints col1 = new ColumnConstraints();
col1.setPercentWidth(50);
ColumnConstraints col2 = new ColumnConstraints();
col2.setPercentWidth(50);
root.getColumnConstraints().addAll(col1,col2);
root.addRow(0, leftCol,rightCol);

Update: In either cases, if you want your buttons to auto stretch, bind the width of the buttons to its layout. This way you can control the buttons width proportion in the HBox.

Button calculate = new Button("Calculate");
Button cancel = new Button("Cancel");
HBox runButtons = new HBox(calculate, cancel);
calculate.prefWidthProperty().bind(runButtons.widthProperty().divide(2));
cancel.prefWidthProperty().bind(runButtons.widthProperty().divide(2));

Update 2: Please find below a sample demo.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.*;
import javafx.stage.Stage;

public class Sample extends Application {

    public void start(Stage stage) throws Exception {
        String blackBorder = "-fx-border-style: solid; -fx-border-width: 1; -fx-border-color: black";

        Button calculate = new Button("Calculate");
        Button cancel = new Button("Cancel");
        HBox runButtons = new HBox(calculate, cancel);
        calculate.prefWidthProperty().bind(runButtons.widthProperty().divide(2));
        cancel.prefWidthProperty().bind(runButtons.widthProperty().divide(2));
        VBox rightCol = new VBox(runButtons);
        rightCol.setStyle(blackBorder);

        Button save = new Button("Save");
        Button del = new Button("Delete");
        HBox settings = new HBox(save, del);
        save.prefWidthProperty().bind(settings.widthProperty().divide(3)); // 1/3
        del.prefWidthProperty().bind(settings.widthProperty().divide(3).multiply(2)); // 2/3
        VBox leftCol = new VBox(settings);
        leftCol.setStyle(blackBorder);


        GridPane root = new GridPane();
        ColumnConstraints col1 = new ColumnConstraints();
        col1.setPercentWidth(50);
        ColumnConstraints col2 = new ColumnConstraints();
        col2.setPercentWidth(50);
        root.getColumnConstraints().addAll(col1,col2);
        root.addRow(0, leftCol,rightCol);

        Scene scene = new Scene(root, 800, 600);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String... a) {
        Application.launch(a);
    }
}