[Java] panel in panel

import java.awt.Color;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;

class TreeMaker extends JPanel
{
	private static final long serialVersionUID = 1;

	public TreeMaker(int i)
	{
	}
}

public class Test
{
	public static void main(String...args)
	{
		JComboBox<Object> test = new JComboBox<Object>();
		test.setBounds(10, 50, 30, 30);
		
		TreeMaker panel = new TreeMaker(5);
		panel.setLayout(null);
		panel.add(test);
		panel.setBackground(Color.GREEN);
		panel.setBounds(10, 10, 100, 100);
	
		JPanel panel1 = new JPanel();
		panel1.setLayout(null);
		panel1.add(panel);
		panel1.setBackground(Color.RED);
	
		JFrame frame = new JFrame("OvalPaint");
		frame.setSize(300, 200);
		frame.add(panel1);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setVisible(true);
	}
}