JavaFX: Ejemplo usando FXML

Controller.java

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;


public class Controller {

    @FXML
    private TextField numero1TextField;
    @FXML
    private TextField numero2TextField;
    @FXML
    private Label resultadoLabel;


    public void sumar(ActionEvent actionEvent) {

        System.out.println("sumar");

        int numero1 = Integer.parseInt(numero1TextField.getText());
        int numero2 = Integer.parseInt(numero2TextField.getText());

        int resultado = numero1 + numero2;

        resultadoLabel.setText(resultado + "");
    }
}

Main.java

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Una suma");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.show();
    }


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

sample.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.ColumnConstraints?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.RowConstraints?>
<?import javafx.scene.layout.VBox?>

<GridPane alignment="center" hgap="10" vgap="10" xmlns="http://javafx.com/javafx/8.0.112" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
    <columnConstraints>
        <ColumnConstraints />
    </columnConstraints>
    <rowConstraints>
        <RowConstraints />
    </rowConstraints>
    <children>
        <VBox prefHeight="200.0" prefWidth="300.0" spacing="20">
            <padding>
                <Insets bottom="20" left="20" right="20" top="20" />
            </padding>
            <children>
                <TextField fx:id="numero1TextField" promptText="Número 1" />
                <TextField fx:id="numero2TextField" promptText="Número 2" />
                <Button mnemonicParsing="false" onAction="#sumar" text="Sumar" />
            <Label text="Resultado" fx:id="resultadoLabel" />
            </children>
            <opaqueInsets>
                <Insets />
            </opaqueInsets>
        </VBox>
    </children>
</GridPane>