java - 在java小程序中加载面板内容

Question

我正在尝试将面板加载到 java 小程序中，但面板的内容未填充。从下面的代码中可以看到，我设置了一个测试来查看面板中的代码在哪里无法运行，测试结果表明 getRootPane().add(MyLabel)是触发异常的代码行。

下面包含重现此问题所需的所有代码。谁能告诉我如何更改下面的代码，以便将面板的内容加载到小程序中？

这是 TestApplet.java 的代码:

import javax.swing.JApplet;
import javax.swing.SwingUtilities;

public class TestApplet extends JApplet {

    public void init(){//Called when this applet is loaded into the browser.
        //Execute a job on the event-dispatching thread; creating this applet's GUI.
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    createGUI();
                }
            });
        } catch (Exception e) {
            System.err.println("createGUI didn't complete successfully");
        }
    }
    private void createGUI(){
        TestPanel myPanel = new TestPanel();
        myPanel.setOpaque(true);
        setContentPane(myPanel);
    }
}

这是 TestPanel.java 的代码:

import javax.swing.JLabel;
import javax.swing.JPanel;

public class TestPanel extends JPanel{

    TestPanel(){
        System.out.println("Running in constructor.  ");
        JLabel myLabel = new JLabel("Hello World");
        getRootPane().add(myLabel);
        System.out.println("Still running in constructor.  ");
    }
}

编辑:

根据目前给出的建议，我编辑了代码如下。使用 this.add 确实会导致 JLabel 加载，但是，内部类仍未加载，我已将其添加到下面的代码中。此外，下面更改后的代码不再触发异常；它只加载 JLabel 但不加载内部类。关于如何加载内部类有什么建议吗？

这是新的 TestApplet.java:

import javax.swing.JApplet;
import javax.swing.SwingUtilities;

public class TestApplet extends JApplet {

    public void init(){//Called when this applet is loaded into the browser.
        //Execute a job on the event-dispatching thread; creating this applet's GUI.
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    createGUI();
                }
            });
        } catch (Exception e) {
            System.err.println("createGUI didn't complete successfully");
            System.err.println(e);
            e.printStackTrace();
        }
    }
    private void createGUI(){
        TestPanel myPanel = new TestPanel();
        myPanel.setOpaque(true);
        setContentPane(myPanel);
    }
}  

这是新的 TestPanel.java:

import java.awt.Canvas;  
import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JLabel;
import javax.swing.JPanel;

public class TestPanel extends JPanel{
    DrawingLines myDrawingLines = new DrawingLines();  

    TestPanel(){
        System.out.println("Running in constructor.  ");
        JLabel myLabel = new JLabel("Hello World");
        this.add(myLabel);
    this.add(myDrawingLines);  
    myDrawingLines.repaint();  
        System.out.println("Still running in constructor.  ");
    }

//inner class to override paint method
class DrawingLines extends Canvas{
   int width, height;

   public void paint( Graphics g ) {
      width = getSize().width;
      height = getSize().height;
      g.setColor( Color.green );
      for ( int i = 0; i < 10; ++i ) {
         g.drawLine( width, height, i * width / 10, 0 );
      }  
      System.out.println("Running in paint method.");  
   }
}//end of inner class   
}  

Answer 1

设置方法为:

 private void createGUI(){
        TestPanel myPanel = new TestPanel();
        getContentPane().add(myPanel);
    }

TestPanel 类为

public class TestPanel extends JPanel{
    TestPanel(){
        super();
        System.out.println("Running in constructor.  ");
        JLabel myLabel = new JLabel("Hello World");
        add(myLabel);
        System.out.println("Still running in constructor.  ");
    }
}