frame.add(child);
這樣子級將被新增到contentPane面板上。內容窗格始終是非 null 的。試圖將其設定為 null 會導致 JFrame 丟擲異常。預設的內容窗格上會設定有BorderLayout管理器。

通常我們在new JFrame()例項的時候，這個頂級容器就為我們分配了兩部分的內容，即：Menu Bar和contentPane.
如下圖：

/**
 * <br>
 * do what you want to do and never stop it.
 * <br>
 */
package com.infy.basecompent;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenuBar;

/**
 * @author Jack
 * 2012-8-17
 * <br>
 */
public class TopLevelDemo {
	/** 
     * Create the GUI and show it.  For thread safety, 
     * this method should be invoked from the 
     * event-dispatching thread. 
     */
    private static void createAndShowGUI() { 
        //Create and set up the window. 
        JFrame frame = new JFrame("TopLevelDemo"); 
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
  
        //Create the menu bar.  Make it have a green background. 
        JMenuBar greenMenuBar = new JMenuBar(); 
        greenMenuBar.setOpaque(true);
        greenMenuBar.setBackground(new Color(154, 165, 127));
        greenMenuBar.setPreferredSize(new Dimension(200, 20));
  
        //Create a yellow label to put in the content pane. 
        JLabel yellowLabel = new JLabel(); 
        yellowLabel.setOpaque(true); 
        yellowLabel.setBackground(new Color(248, 213, 131)); 
        yellowLabel.setPreferredSize(new Dimension(200, 180)); 
  
        //Set the menu bar and add the label to the content pane. 
        frame.setJMenuBar(greenMenuBar); 
        frame.getContentPane().add(yellowLabel, BorderLayout.CENTER); 
  
        //Display the window. 
        frame.pack(); 
        frame.setVisible(true); 
    }

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// Schedule a job for the event-dispatching thread:
		// creating and showing this application's GUI.
		javax.swing.SwingUtilities.invokeLater(new Runnable() {
			public void run() {
				createAndShowGUI();
			}
		});
	}

}