问为什么用Java的JFileChooser不能双击选择文件？

EN

import java.awt.*;
import java.awt.event.*;
import java.io.File;
import javax.swing.filechooser.FileFilter;

import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;

public class Demo_File_Chooser_Show_Txt_Only extends JFrame
{
  private JButton buttonBrowse;

  public Demo_File_Chooser_Show_Txt_Only()
  {
    super("Demo File Type Filter");
    setLayout(new FlowLayout());
    buttonBrowse=new JButton("Browse...");
    buttonBrowse.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent arg0) { showOpenFileDialog(); } });
    getContentPane().add(buttonBrowse);
    setSize(300,100);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setVisible(true);
  }

  public static void main(String[] args)
  {
    try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); }
    catch (Exception e) { }
    SwingUtilities.invokeLater(new Runnable() { public void run() { new Demo_File_Chooser_Show_Txt_Only(); } });
  }

  private void showOpenFileDialog()
  {
    JFileChooser fileChooser=new JFileChooser(".");
    fileChooser.setControlButtonsAreShown(false);
    fileChooser.setFileFilter(new FileFilter()
    {
      @Override
      public boolean accept(File file) { return file.getName().toLowerCase().endsWith(".txt"); }

      @Override
      public String getDescription() { return ".txt files"; }
    });
//    fileChooser.setFileSelectionMode(JFileChooser.DIRECTORIES_ONLY);
//    fileChooser.setFileSelectionMode(JFileChooser.FILES_ONLY);
    fileChooser.showOpenDialog(null);
  }
}