JAVA编写GUI界面时如何用一个新的JTree对象将之前的JTree对象覆盖掉？

tree是一个成员变量，我先写了个方法生成了一个空的树，然后想在更新树的方法里给tree重新赋值来覆盖掉原先的空树，可是运行时覆盖不掉，那个树还是显示为空。就想问一下如何才能覆盖之前的树以达到更新树的目的？

//生成树
public void layoutTreePanel() {
DefaultMutableTreeNode initTree = new DefaultMutableTreeNode(null);
tree = new JTree(TreePath);
setTree(path,TreePath);
JScrollPane scrollPane = new JScrollPane();
scrollPane.setViewportView(tree);
treePanel.add(scrollPane,BorderLayout.CENTER);

}

//更新树
public void showTree(String str) {
    String path = str;
    DefaultMutableTreeNode TreePath = new DefaultMutableTreeNode(path);
    tree = new JTree(TreePath);
    JScrollPane scrollPane = new JScrollPane();
    scrollPane.setViewportView(tree);
    treePanel.add(scrollPane,BorderLayout.CENTER);

}