Why FlowLayout - JPane will NOT auto adjust in size?

Question

Hello java friends,
I don't understand why my little demo program's JPane(use FlowLayout manager) will not adjust the size correctly. When run, I see one row of buttons, which only show 11 of them; the rest are shown on secon row which only visible if I go resize the window. Why is that? Wouldn't FlowLayout manager will auto size the pane? My main frame used the pack() method, so it's up to the pane's preferredsize. Please give me some ideas. Thanks.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Template {
    public static void main(String[] args) {
         JFrame mainFrame =new JFrame("Template");
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.setContentPane(new TemplatePane());
        //Display the window.
        mainFrame.pack();
        mainFrame.setVisible(true);
class TemplatePane extends JPanel
          implements ActionListener {
     JButton componentAry[] =new JButton[100];
     TemplatePane() {
          this.setLayout(new FlowLayout());
          for(int i=0; i<componentAry.length; i++) {
               componentAry[i] =new JButton("Test#"+i);
               componentAry.addActionListener(this);
               this.add(componentAry[i]);
     public void actionPerformed(ActionEvent ae) {
          System.out.println(ae.getActionCommand());

Answer 1

I did read the API doc:
>>>>
A flow layout arranges components in a left-to-right flow, much like lines of text in a paragraph. Flow layouts are typically used to arrange buttons in a panel. It will arrange buttons left to right until no more buttons fit on the same line. Each line is centered.
>>>>
My question is why won't it adjust the pane size if the there is more than one line? It did draw the second line for things that won't fit in the first line... just not visible unless the whole window is resized to a bigger view. Can you please provide an explanation?