Java 加载包含另一个JPanel的JPanel

我正在尝试加载一个扩展JPanel的类，该类包含来自另一个类的组件，该类也将JPanel扩展到另一个类中。

这就是我需要实现的目标：

First.java的示例

  public class First extends JPanel{
      JPanel cont = new JPanel();
          public First(){
          cont.setBackground(Color.YELLOW);
          }
       }

  public class Second extends JPanel{
      JPanel cont = new JPanel();
      First first_panel = new First();
          public Second(){
          cont.setBackground(Color.RED);
          cont.add(first_panel);
          }
      }

   public class Container extends JFrame{
       JFrame frame = new JFrame();
       JPanel cont = new JPanel();
       Second second_panel = new Second();
          public Container(){
          cont.setBackground(Color.GREEN);
          cont.add(second_panel);
          frame.add(cont);
          frame.setVisible(true);
          }
      }

Second.java的示例

  public class First extends JPanel{
      JPanel cont = new JPanel();
          public First(){
          cont.setBackground(Color.YELLOW);
          }
       }

  public class Second extends JPanel{
      JPanel cont = new JPanel();
      First first_panel = new First();
          public Second(){
          cont.setBackground(Color.RED);
          cont.add(first_panel);
          }
      }

   public class Container extends JFrame{
       JFrame frame = new JFrame();
       JPanel cont = new JPanel();
       Second second_panel = new Second();
          public Container(){
          cont.setBackground(Color.GREEN);
          cont.add(second_panel);
          frame.add(cont);
          frame.setVisible(true);
          }
      }

Container.java的示例

  public class First extends JPanel{
      JPanel cont = new JPanel();
          public First(){
          cont.setBackground(Color.YELLOW);
          }
       }

  public class Second extends JPanel{
      JPanel cont = new JPanel();
      First first_panel = new First();
          public Second(){
          cont.setBackground(Color.RED);
          cont.add(first_panel);
          }
      }

   public class Container extends JFrame{
       JFrame frame = new JFrame();
       JPanel cont = new JPanel();
       Second second_panel = new Second();
          public Container(){
          cont.setBackground(Color.GREEN);
          cont.add(second_panel);
          frame.add(cont);
          frame.setVisible(true);
          }
      }

我可以逐个加载类，但当我尝试加载包含另一个面板的面板时，GUI不会显示它。逻辑有什么问题？问题出在哪里？
显示的代码中有两个基本问题

每个类都进行了扩展，并且都有它所处理的组件的实例
两个面板都没有任何内容可以赋予它非零大小，也没有覆盖
getPreferredSize
方法，因此它们是0x0 px
看看这个MCVE的效果

import java.awt.Color;
import javax.swing.*;

public class Container {

    JFrame frame = new JFrame();
    JPanel cont = new JPanel();
    Second second_panel = new Second();

    public Container() {
        cont.setBackground(Color.GREEN);
        cont.add(second_panel.getPanel());
        frame.add(cont);
        frame.pack();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {

            @Override
            public void run() {
                new Container();
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

class Second {

    JPanel cont = new JPanel();
    First first_panel = new First();

    public Second() {
        cont.setBackground(Color.RED);
        cont.add(new JLabel("Second"));
        cont.add(first_panel.getPanel());
    }

    public JComponent getPanel() {
        return cont;
    }
}

class First {

    JPanel cont = new JPanel();

    public First() {
        cont.setBackground(Color.YELLOW);
        cont.add(new JLabel("First"));
    }

    public JComponent getPanel() {
        return cont;
    }
}


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