标签不是在另一个之下动态创建的

Question

http://ibb.co/hvrdbR private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
//删除现有面板 字符串 xmldoc="./src/com/ui/Data.xml"; 尝试{ JLabel label2=new JLabel();
文件 fXmlFile = 新文件（xmldoc）； DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance(); DocumentBuilder dBuilder = dbFactory.newDocumentBuilder(); 文档 doc = dBuilder.parse(fXmlFile); doc.getDocumentElement().normalize(); NodeList nList = doc.getElementsByTagName("username"); for (int temp = 0; temp jPanel7.setBackground(Color.WHITE); jPanel7.setLayout(new FlowLayout()); jPanel7.add(label2); label2.setText(文本); label2.setPreferredSize(新维度(100,40)); label2.setFont(new Font("Times New Roman", Font.BOLD, 24));
} } }
捕捉（异常前）{ System.out.println("数据库异常：userExists()"); } }

只创建标签并粘贴所有用户名

Answer 1

在循环的每次迭代中，jPanel7.add(label2) 被调用一次。但是，您一遍又一遍地使用相同的 JLabel 实例。

接下来会发生什么？

JPanel.add(Component) 在java.awt.Container.add(Component) 中定义，它被委托给java.awt.Container.addImpl(Component, Object, int)。并在代码中：

 /* Reparent the component and tidy up the tree's state. */
 if (comp.parent != null) {
      comp.parent.remove(comp);
      if (index > component.size()) {
           throw new IllegalArgumentException("illegal component position");
      }
 }
 // ...
 comp.parent = this;

尝试将Component 添加到已包含此Component 的Container 将导致实际操作中不会发生任何事情。因此，最后你的jPanel7只有一个孩子——label2。

什么是正确的做法？

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    String xmldoc="./src/com/ui/Data.xml";
    try {
        // background and layout should be set only once
        jPanel7.setBackground(Color.WHITE);
        jPanel7.setLayout(new FlowLayout());

        File fXmlFile = new File(xmldoc);
        DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
        Document doc = dBuilder.parse(fXmlFile);
        doc.getDocumentElement().normalize();
        NodeList nList = doc.getElementsByTagName("username");

        for (int temp = 0; temp < nList.getLength(); temp++) {
            Node nNode = nList.item(temp);
            if (nNode.getNodeType() == Node.ELEMENT_NODE) {
                Element eElement = (Element) nNode;
                String text = eElement.getTextContent();
                System.out.println(text);

                JLabel label2 = new JLabel(text);  // create a new JLabel with the given text
                // set label2's attributes
                label2.setPreferredSize(new Dimension(100,40));
                label2.setFont(new Font("Times New Roman", Font.BOLD, 24)); 

                jPanel7.add(label2);  // add it to jPanel7
            } 
        }
    } catch (Exception ex) {
        System.out.println("Database exception : userExists()");
    }
}