JavaFX - 如何获得节点首选大小

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

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

    @Override
    public void start(Stage stage) {
        Button okBtn = new Button("OK");
        Button cancelBtn = new Button("Cancel");

        cancelBtn.setPrefWidth(100);

        VBox root = new VBox();
        root.getChildren().addAll(okBtn, cancelBtn);

        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.setTitle("Overriding Node Sizes");
        stage.show();

        System.out.println("okBtn.getPrefWidth(): " + okBtn.getPrefWidth());
        System.out.println("okBtn.getMinWidth(): " + okBtn.getMinWidth());
        System.out.println("okBtn.getMaxWidth(): " + okBtn.getMaxWidth());

        System.out.println("cancelBtn.getPrefWidth(): " + cancelBtn.getPrefWidth());
        System.out.println("cancelBtn.getMinWidth(): " + cancelBtn.getMinWidth());
        System.out.println("cancelBtn.getMaxWidth(): " + cancelBtn.getMaxWidth());
    }
}