JavaFX HBox create a simple calculator

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

public class Main extends Application {

   double number1 = 0;
   double number2 = 0;
   double answer = 0;

   @Override// w w  w.j a v  a  2s  . co  m
   public void start(Stage primaryStage) throws Exception {

      HBox numberPane = new HBox();
      numberPane.setSpacing(10);
      numberPane.setAlignment(Pos.CENTER);

      Label lblNumber1 = new Label("Number 1:");
      TextField tfNumber1 = new TextField();

      Label lblNumber2 = new Label("Number 2:");
      TextField tfNumber2 = new TextField();

      Label lblResult = new Label("Result :");
      TextField tfResult = new TextField();

      numberPane.getChildren().addAll(lblNumber1, tfNumber1, lblNumber2, tfNumber2, lblResult, tfResult);

      Button btAdd = new Button("Add");
      btAdd.setOnAction(e -> {
         answer = Double.parseDouble(tfNumber1.getText()) + Double.parseDouble(tfNumber2.getText());
         tfResult.setText(answer + "");
      });

      Button btSubtract = new Button("subtract");
      btSubtract.setOnAction(e -> {
         answer = Double.parseDouble(tfNumber1.getText()) - Double.parseDouble(tfNumber2.getText());
         tfResult.setText(answer + "");
      });
      Button btMultiply = new Button("Multiply");
      btMultiply.setOnAction(e -> {
         answer = Double.parseDouble(tfNumber1.getText()) * Double.parseDouble(tfNumber2.getText());
         tfResult.setText(answer + "");
      });
      Button btDivide = new Button("Divide");
      btDivide.setOnAction(e -> {
         answer = Double.parseDouble(tfNumber1.getText()) / Double.parseDouble(tfNumber2.getText());
         tfResult.setText(answer + "");
      });

      HBox operatorsPane = new HBox();
      operatorsPane.setSpacing(10);
      operatorsPane.setAlignment(Pos.CENTER);
      operatorsPane.getChildren().addAll(btAdd, btSubtract, btMultiply, btDivide);

      BorderPane borderPane = new BorderPane(numberPane);
      BorderPane.setMargin(numberPane, new Insets(10, 10, 10, 10));

      borderPane.setBottom(operatorsPane);
      BorderPane.setMargin(operatorsPane, new Insets(10, 10, 10, 10));
      primaryStage.setScene(new Scene(borderPane, borderPane.getPrefWidth(), borderPane.getPrefWidth()));
      primaryStage.setTitle("Simple calculator");
      primaryStage.show();
   }

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