class TreeNodeFactory {

    /**
     * The factory that can produce all indirect objects.
     */
    protected IndirectObjectFactory objects;
    /**
     * An list containing the nodes of every indirect object.
     */
    protected ArrayList<PdfObjectTreeNode> nodes = new ArrayList<>();

    /**
     * Creates a factory that can produce TreeNode objects corresponding with PDF objects.
     *
     * @param objects a factory that can produce all the indirect objects of a PDF file.
     */
    public TreeNodeFactory(IndirectObjectFactory objects) {
        this.objects = objects;
        for (int i = 0; i < objects.size(); i++) {
            int ref = objects.getRefByIndex(i);
            nodes.add(PdfObjectTreeNode.getInstance(PdfNull.PDFNULL, ref));
        }
    }

    /**
     * Gets a TreeNode for an indirect objects.
     *
     * @param ref the reference number of the indirect object.
     * @return the TreeNode representing the PDF object
     */
    public PdfObjectTreeNode getNode(int ref) {
        int idx = objects.getIndexByRef(ref);
        PdfObjectTreeNode node = nodes.get(idx);
        if (node.getPdfObject().isNull()) {
            node = PdfObjectTreeNode.getInstance(objects.loadObjectByReference(ref), ref);
            nodes.set(idx, node);
        }
        return node;
    }

    /**
     * Creates the Child TreeNode objects for a PDF object TreeNode.
     *
     * @param node the parent node
     */
    public void expandNode(PdfObjectTreeNode node) {<FILL_FUNCTION_BODY>}

    /**
     * Finds a specific child of dictionary node.
     *
     * @param node the node with a dictionary among its children
     * @param key  the key of the item corresponding with the node we need
     * @return the PdfObjectTreeNode that is the child of the dictionary node
     */
    public PdfObjectTreeNode getChildNode(PdfObjectTreeNode node, PdfName key) {
        Enumeration children = node.breadthFirstEnumeration();
        PdfObjectTreeNode child;
        while (children.hasMoreElements()) {
            child = (PdfObjectTreeNode) children.nextElement();
            if (child.isDictionaryNode(key)) {
                if (child.isIndirectReference()) {
                    expandNode(child);
                    child = (PdfObjectTreeNode) child.getFirstChild();
                }
                expandNode(child);
                return child;
            }
        }
        return null;
    }

    /**
     * Tries adding a child node to a parent node without throwing an exception. Normally, if the child node is already
     * added as one of the ancestors, an IllegalArgumentException is thrown (to avoid an endless loop). Loops like this
     * are allowed in PDF, not in a JTree.
     *
     * @param parent the parent node
     * @param child  a child node
     */
    private void addNodes(PdfObjectTreeNode parent, PdfObjectTreeNode child) {
        try {
            parent.add(child);
        } catch (IllegalArgumentException iae) {
            parent.setRecursive(true);
        }
    }
}

        if (node.getChildCount() > 0) {
            return;
        }
        PdfObject object = node.getPdfObject();
        PdfObjectTreeNode leaf;
        switch (object.type()) {
            case PdfObject.INDIRECT:
                PdfIndirectReference ref = (PdfIndirectReference) object;
                leaf = getNode(ref.getNumber());
                addNodes(node, leaf);
                if (leaf instanceof PdfPagesTreeNode) {
                    expandNode(leaf);
                }
                return;
            case PdfObject.ARRAY:
                PdfArray array = (PdfArray) object;
                for (PdfObject pdfObject : array.getElements()) {
                    leaf = PdfObjectTreeNode.getInstance(pdfObject);
                    addNodes(node, leaf);
                    expandNode(leaf);
                }
                return;
            case PdfObject.DICTIONARY:
            case PdfObject.STREAM:
                PdfDictionary dict = (PdfDictionary) object;
                for (PdfName pdfName : dict.getKeys()) {
                    leaf = PdfObjectTreeNode.getInstance(dict, pdfName);
                    addNodes(node, leaf);
                    expandNode(leaf);
                }
                return;
        }
    
<no_super_class>