Underscore causing an exception in a toolbar

Question

//  I'm using Netbeans 7.4 release candidtate 2
//  1)  Start the JavaFx Ensemble program - thank you very much.  It has saved me many hours.
//  2)  In Ensemble Click on "Hidden Split Pane".
//  3)  Click the "Source Code" tab.
//  4)  Click the "Save Netbeans project".
//  5)  Replace HiddenSplitPaneSample.java with this code (see below).
//  6)  After starting: click the "double arrow" twice (maybe three times) to display the options in the center pane.
//  7)  The second time you should get the exception.  (See Netbeans output window.)
//  8)  You get the double arrow because dividerPositions() is set too small; but that may be the way some user
//      would have things "split".
//  Why this is important to me.  I included a phony getWord() method (see below).  In my app getWord() will
//  actually go to disk and get a word, in the human language that the user has selected somewhere else.
//  Example:    if they have English selected then getWord( "Add_One" ) will return "Add One" and display it as the buttons text.
//              if they have Chinese selected then getWord( "Add_One" ) will return "Chinese equivalent of Add One" and display it as the buttons text.
//              If there is no replacement in the file then getWord( "Add_One" ) will return "Add_One".
//  The question is why does the underscore cause an exception?  I could easily replace the under score with
//  a dash (see below) because that doesn't cause the exception. But that begs the question what will happen
//  when I'm using Chinese, Japanese, or Hindi?  Is there a stray character here and there that will cause
//  this exception?
//  Notice also that it has removed my underscore: "Add_One" becomes "AddOne".
* Copyright (c) 2008, 2012 Oracle and/or its affiliates.
* All rights reserved. Use is subject to license terms.
import javafx.application.Application;
import javafx.geometry.HPos;
import javafx.geometry.VPos;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ScrollPane;
import javafx.stage.Stage;
import javafx.scene.control.SplitPane;
import javafx.scene.control.SplitPaneBuilder;
import javafx.scene.control.ToolBar;
import javafx.scene.control.ToolBarBuilder;
import javafx.scene.layout.Pane;
import javafx.scene.layout.RegionBuilder;
import javafx.scene.layout.VBox;
public class HiddenSplitPaneSample extends Application {
    ToolBar     standardToolbar     = null;
    ScrollPane  scrollPane          = new ScrollPane    ();
    class MyPane extends Pane   {
        @Override
        protected void layoutChildren() {
            super.layoutChildren   ();
            double  w   = getWidth  ();
            double  h   = getHeight ();
            double  ph  = standardToolbar.getHeight();
            layoutInArea    ( standardToolbar,  0,  0,  w, ph,    0,  HPos.CENTER,    VPos.CENTER );
            layoutInArea    ( scrollPane,       0,  0+ph,   w, h-ph,    0,  HPos.LEFT,    VPos.CENTER );
    private String  getWord ( String wordToGet )    {
        return  wordToGet;
    private void init(Stage primaryStage) {
        MyPane  pane1   = new MyPane    ();
                pane1   .setPrefWidth   ( 400 );
                pane1   .setPrefHeight  ( 400 );
        standardToolbar = ToolBarBuilder.create().build();
            standardToolbar     .setPrefHeight      ( 30 );
            standardToolbar     .setMinHeight       ( 30 );
            standardToolbar     .setMaxHeight       ( 30 );
            pane1.getChildren() .add( standardToolbar );
            Button  addOneButton    = new Button    ( getWord( "Add_One"    ) );
            Button  addTwoButton    = new Button    ( getWord( "Add_Two"    ) ); 
            Button  addThreeButton  = new Button    ( getWord( "Add_Three"  ) );
            Button  addFourButton   = new Button    ( getWord( "Add_Four"   ) ); 
            //Button  addOneButton    = new Button    ( getWord( "Add-One"    ) );
            //Button  addTwoButton    = new Button    ( getWord( "Add-Two"    ) ); 
            //Button  addThreeButton  = new Button    ( getWord( "Add-Three"  ) );
            //Button  addFourButton   = new Button    ( getWord( "Add-Four"   ) ); 
            standardToolbar .getItems().add ( addOneButton      );
            standardToolbar .getItems().add ( addTwoButton      );
            standardToolbar .getItems().add ( addThreeButton    );
            standardToolbar .getItems().add ( addFourButton     );
        VBox    vbox    = new VBox  ();
        scrollPane  .setContent     ( vbox );
        scrollPane  .setFitToWidth  ( true );
        pane1   .getChildren().add     ( scrollPane );
        Group root = new Group();
        primaryStage.setScene(new Scene(root));
        String hidingSplitPaneCss = HiddenSplitPaneSample.class.getResource("HiddenSplitPane.css").toExternalForm();
        final SplitPane splitPane = SplitPaneBuilder.create().id("hiddenSplitter").items(
                RegionBuilder.create().styleClass("rounded").build(),
                pane1,//RegionBuilder.create().styleClass("rounded").build(),
                RegionBuilder.create().styleClass("rounded").build()).dividerPositions(new double[]{0.33, 0.50}).build();
        splitPane.getStylesheets().add(hidingSplitPaneCss);
        root.getChildren().add(splitPane);
    @Override public void start(Stage primaryStage) throws Exception {
        primaryStage.setX      ( 0 );
        primaryStage.setY      ( 0 );
        primaryStage.setWidth  ( 500 );
        primaryStage.setHeight ( 500 );
        init(primaryStage);
        primaryStage.show();
    public static void main(String[] args) { launch(args); }

Answer 1

The underscore is being parsed as an indicator that the next character should be a mnemonic, and the underscore is then stripped. This happens by default on Buttons, but not on Labels.
See the API in the Labeled class. You can either turn parsing off, or you can use two underscores to represent one underscore.