public class Main extends Application {

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


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

}

public class Controller {
    public void open(ActionEvent actionEvent) {
        Parent root = null;
        try {
            FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("window.fxml"));
            Parent root1 = (Parent) fxmlLoader.load();
            Stage stage = new Stage();
            stage.setTitle("ABC");
            stage.setScene(new Scene(root1, 300, 275));
            stage.show();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void test(ActionEvent actionEvent) {
        Alert alert = new Alert(Alert.AlertType.INFORMATION);
        alert.setTitle("Information Dialog");
        alert.setHeaderText("Look, an Information Dialog");
        alert.setContentText("I have a great message for you!");
        alert.showAndWait();
    }
}

<?import javafx.scene.control.Button?>
<?import javafx.scene.layout.*?>
<GridPane fx:controller="sample.Controller"
          xmlns:fx="http://javafx.com/fxml" alignment="center" hgap="10" vgap="10">
    <columnConstraints>
        <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
        <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
    </columnConstraints>
    <children>
    <Button onAction="#open" text="Open">
    </Button>

    <Button onAction="#test" text="Test" GridPane.columnIndex="1"/>
    </children>
</GridPane>

<?import javafx.scene.layout.GridPane?>

<?import javafx.scene.control.Button?>

<GridPane fx:controller="sample.Controller"
          xmlns:fx="http://javafx.com/fxml" alignment="center" hgap="10" vgap="10">

</GridPane>