我的jTree没有显示节点

Question

我正在使用jTree，当我设置模型时，jTree不显示更改，代码是下一个

public class Main extends javax.swing.JFrame {

    public Main() {
        initComponents();
        treeExample();
    }


    @SuppressWarnings("unchecked")
    private void initComponents() {
       ...
    }

    public void treeExample(){
       DefaultMutableTreeNode top =
        new DefaultMutableTreeNode("The Java Series");
        createNodes(top);
        jTree1 = new JTree(top);
        jScrollPane1 = new JScrollPane(jTree1);  
    }
private void createNodes(DefaultMutableTreeNode top) {
    DefaultMutableTreeNode category = null;
    DefaultMutableTreeNode book = null;

    category = new DefaultMutableTreeNode("Books for Java Programmers");
    top.add(category);

    book = new DefaultMutableTreeNode
       ("The Java Tutorial: A Short Course on the Basics");
    category.add(book);

}
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JTree jTree1;
}

我直接使用desing模式，可能是什么问题？