关于JFrame,JPane及其它容器的层次关系？？？？

/*
* SwingApplication.java is a 1.4 example that requires
* no other files.
*/
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class SwingApplication implements ActionListener {
private static String labelPrefix = "Number of button clicks: ";
private int numClicks = 0;
final JLabel label = new JLabel(labelPrefix + "0 ");

//Specify the look and feel to use. Valid values:
//null (use the default), "Metal", "System", "Motif", "GTK+"
final static String LOOKANDFEEL = null; //Mark 1

public Component createComponents() {　//Component类与JFrame类的层次关系？？？
JButton button = new JButton("I'm a Swing button!");
button.setMnemonic(KeyEvent.VK_I);
button.addActionListener(this);
label.setLabelFor(button);//That code exists solely as a hint to assistive technologies,

//such as screen readers, that the label describes the button.

//what is the sceen readers??

/*
* An easy way to put space between a top-level container //为什么不能直接加上了？？？非得先加到JPane上么？？？ ＮＯＴＥ3
* and its contents is to put the contents in a JPanel
* that has an "empty" border.
*/
JPanel pane = new JPanel(new GridLayout(0, 1));　　
pane.add(button);
pane.add(label);
pane.setBorder(BorderFactory.createEmptyBorder(
30, //top
30, //left
10, //bottom
30) //right
);

return pane;
}

public void actionPerformed(ActionEvent e) {
numClicks++;
label.setText(labelPrefix + numClicks);
}

private static void initLookAndFeel() { //NOTE 2
String lookAndFeel = null;

if (LOOKANDFEEL != null) {　　//LOOKANDFEEL在哪定义???　见上面标注1

//不过我不明白，既然LOOKANDFEEL已定义为final,且同时也赋值为null　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　那还有什么可比较的呢？？？？？
if (LOOKANDFEEL.equals("Metal")) {
lookAndFeel = UIManager.getCrossPlatformLookAndFeelClassName();
} else if (LOOKANDFEEL.equals("System")) {
lookAndFeel = UIManager.getSystemLookAndFeelClassName();
} else if (LOOKANDFEEL.equals("Motif")) {
lookAndFeel = "com.sun.java.swing.plaf.motif.MotifLookAndFeel";
} else if (LOOKANDFEEL.equals("GTK+")) { //new in 1.4.2
lookAndFeel = "com.sun.java.swing.plaf.gtk.GTKLookAndFeel";
} else {
System.err.println("Unexpected value of LOOKANDFEEL specified: "
+ LOOKANDFEEL);
lookAndFeel = UIManager.getCrossPlatformLookAndFeelClassName();
}

try {
UIManager.setLookAndFeel(lookAndFeel);
} catch (ClassNotFoundException e) {
System.err.println("Couldn't find class for specified look and feel:"
+ lookAndFeel);
System.err.println("Did you include the L&F library in the class path?");
System.err.println("Using the default look and feel.");
} catch (UnsupportedLookAndFeelException e) {
System.err.println("Can't use the specified look and feel ("
+ lookAndFeel
+ ") on this platform.");
System.err.println("Using the default look and feel.");
} catch (Exception e) {
System.err.println("Couldn't get specified look and feel ("
+ lookAndFeel
+ "), for some reason.");
System.err.println("Using the default look and feel.");
e.printStackTrace();
}
}
}

/**
* Create the GUI and show it. For thread safety,
* this method should be invoked from the
* event-dispatching thread.
*/
private static void createAndShowGUI() {
//Set the look and feel.
initLookAndFeel(); //See NOTE 2

//Make sure we have nice window decorations.
JFrame.setDefaultLookAndFeelDecorated(true);

//Create and set up the window.
JFrame frame = new JFrame("SwingApplication");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

SwingApplication app = new SwingApplication(); //一见到这个心里边得先绕个弯儿呀，总得先回想下，这　是调用此类中那个默认的构造方法，//另外问题：像现在这种情况下，怎么根据上下文来确定字体、颜色等属性呢？？？？
Component contents = app.createComponents(); //第一次见这样用某一方法的返回类型来定义对象。

//createComponents方法返回一个JPane类，而JPane类为Component的子类。可行！！！
frame.getContentPane().add(contents, BorderLayout.CENTER);　//接上面的ＮＯＴＥ3，是不是在JFrame上加了一层JPane后，就可以很方便的应用 BorderLayout.CENTER了这样的功能设置了呢???

//Display the window.
frame.pack(); //现在这个pack()方法的详细描述，它是根据哪些因素来

//类库的解释为：Causes this Window to be sized to fit the preferred size and layouts of its subcomponents. If the window and/or its owner are not yet displayable, both are made displayable before calculating the preferred size. The Window will be validated after the preferredSize is calculated.

//在这儿perferred的标准有哪些？？？？
frame.setVisible(true);
}

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();
}
});
}
}