java - Java 中有 JPanel 限制吗？

Question

在下面的代码中，当 JPanel 未添加到 JFrame 扩展时，程序运行正常，但是，我注释掉了 JPanel 并使用了 GridLayout，所有内容都正确且漂亮地放置在 JFrame 上。 JPanel 显示对象的方式是否有一些限制？在代码中我注释掉了以下语句:

  this.setLayout(new GridLayout(3,2));
   this.add(nameLabel);
   this.add(name);
   this.add(urlLabel);
   this.add(url);
   this.add(typeLabel);
   this.add(type);

程序显示错误，但如果上面的语句未注释，而下面的语句注释:

   //add some panes..
    JPanel panel = new JPanel();
    panel.add(nameLabel);
    panel.add(name);
    panel.add(urlLabel);
    panel.add(url);
    panel.add(typeLabel);
    panel.add(type);
    this.add(panel);

，那么程序就可以正常工作了。下面是完整的代码摘录。

[CODE]

public class FeedInfo extends JFrame {
  private JLabel nameLabel = new JLabel("Name:", SwingConstants.RIGHT);
  private JTextField name;
  private JLabel urlLabel = new JLabel("URL",SwingConstants.RIGHT);
  private JTextField url;
  private JLabel typeLabel = new JLabel("Type",SwingConstants.RIGHT);
  private JTextField type;


   public FeedInfo()
      {
         super("Feed Information");
         this.setSize(400,105);
         this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

         String response1 = JOptionPane.showInputDialog(null,"Enter the site name:");
         name = new JTextField(response1,20);
         String response2 = JOptionPane.showInputDialog(null,"Enter the site address:");
         url = new JTextField(response2, 20);

         String[] choices ={"Personal","Commercial","Unkown"};
         int response3 = JOptionPane.showOptionDialog(null, "what type of site is it?",
           "site Type",0, JOptionPane.QUESTION_MESSAGE, null, choices, choices[0]);

        type = new JTextField(choices[response3],20);
        //add some panes..
        JPanel panel = new JPanel();
        panel.add(nameLabel);
        panel.add(name);
        panel.add(urlLabel);
        panel.add(url);
        panel.add(typeLabel);
        panel.add(type);
        this.add(panel);


        /*this.setLayout(new GridLayout(3,2));
       this.add(nameLabel);
       this.add(name);
       this.add(urlLabel);
       this.add(url);
       this.add(typeLabel);
       this.add(type);
       */

       this.setVisible(true);

    }
[/CODE]

Answer 1

JPanel 本身不执行布局，它使用 LayoutManager。在您的情况下，您想要覆盖默认的 FlowLayout 并使用 GridLayout (就像您对 JFrame 所做的那样)。所以就这样做吧!

JPanel panel = new JPanel(new GridLayout(3, 2));
...