JTree does not show up

Question

Why the JTree does not show up? Here is my code:

    initComponents();
    JTree treeView;
    DefaultMutableTreeNode top = new DefaultMutableTreeNode("myBooks");
    DefaultMutableTreeNode category = new DefaultMutableTreeNode("Drama");
    DefaultMutableTreeNode book1 = new DefaultMutableTreeNode("Macbeth");
    DefaultMutableTreeNode book2 = new DefaultMutableTreeNode("Hamlet");
    category.add(book1);
    category.add(book2);
    top.add(category);
    treeView = new JTree(top);
    JScrollPane pane = new JScrollPane(treeView);
    mainPanel.add(pane);

Answer 1

Give your JTree a background and see what space it occupies on screen. Also you can use CTRL + SHIFT + F1 on a swing app and see the different components in console with size, position and everything.

My guess is that mainPanel doesn't have a good layout or the jtree should have a decent preferred size set.

Answer 2

To extend on Snicolas' answer (1+ to him) you appear to be adding the JScrollPane to your mainPanel without regard for the layout manager used. I'm guessing that your GUI is using the GroupLayout, and if so, I suggest you use a layout manager that is more user friendly. Also, are you adding the pane JScrollPane after pack and setVisible(true) are called on the top-level window? If so you'll need to revalidate and repaint the container that is receiving the new component.