Question

我想绘制由.form创建的子面板。有一个“mainPanel”，它包含三个名为panel1（顶部），panel2（左下角）和panel3（右下角）的子面板。

现在我想在subPanel“panel1”上画一些东西，但它不起作用。当我运行程序时，它只显示那里的三个子面板，但没有我的“paintComponent（）”方法的图纸。

我在这里粘贴了我的代码，有人可以帮我检查一下是什么问题吗？非常感谢。

public class PanelDrawTest extends JFrame {
    private JPanel mainPanel;
    private JPanel panel1;
    private JPanel panel2;
    private JPanel panel3;

    public PanelDrawTest(){
       getContentPane().add(mainPanel);
       setPanel1(new MyPanel1());
    }

    public JPanel getMainPanel() {
       return mainPanel;
    }

    public JPanel getPanel1() {
       return panel1;
    }

    public void setPanel1(JPanel panel1) {
       this.panel1 = panel1;

    }

    private class MyPanel1 extends JPanel {

       public MyPanel1(){

       }
       @Override
       public void paintComponent(Graphics g){
           g.drawString("This is Panel 1",20,20);
           g.drawRect(0,200,100,200);

       }
    }

    public static void main(String args[]){
        SwingUtilities.invokeLater(new Runnable() {
           @Override
           public void run() {
               createAndShowGUI();
           }
        });
    }

    private static void createAndShowGUI() {
        PanelDrawTest frame = new PanelDrawTest();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setBounds(50, 50, 1200, 700);
        frame.setPreferredSize(new Dimension(1200,800));
        frame.pack();
        frame.setVisible(true);
    }

 }

Answer 1

注意：发布的代码会抛出NullPointerException，因为mainPanel从未初始化。

构造函数没有将自定义面板添加到布局中，这就是无法看到它的原因。试试这个，你会看到你的画：

public PanelDrawTest() {
   mainPanel = new JPanel();
   getContentPane().add(mainPanel,BorderLayout.NORTH);
   panel1 = new MyPanel1();
   getContentPane().add(panel1,BorderLayout.CENTER);      
}

Answer 2

您的版本不能为我运行，但这样做有效：

public class PanelDrawTest extends JFrame {
  private JPanel mainPanel = new JPanel(); // I added a constructor to avoid NullPointerexception
  private JPanel panel1;
  private JPanel panel2;
  private JPanel panel3;

  public PanelDrawTest(){
    getContentPane().add(mainPanel);
    setPanel1(new MyPanel1());
    add(panel1); // and I added the panel
  }

  public JPanel getMainPanel() {
     return mainPanel;
  }

  public JPanel getPanel1() {
    return panel1;
  }

  public void setPanel1(JPanel panel1) {
    this.panel1 = panel1;

  }

  private class MyPanel1 extends JPanel {

    public MyPanel1(){

    }
    @Override
    public void paintComponent(Graphics g){
      g.drawString("This is Panel 1",20,20);
      g.drawRect(0,200,100,200);

    }
  }

  public static void main(String args[]){
    SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
        createAndShowGUI();
      }
    });
  }

  private static void createAndShowGUI() {
    PanelDrawTest frame = new PanelDrawTest();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setBounds(50, 50, 1200, 700);
    frame.setPreferredSize(new Dimension(1200,800));
    frame.pack();
    frame.setVisible(true);
  }
}

如何绘制.form创建的子面板？

2 个答案: