大家帮忙看看，小弟我这个代码中，最后写的那个注释那个地方。为何一次选择，执行两次该事件？

import java.awt.*;import javax.swing.*;import javax.swing.event.*;import javax.swing.event.ListSelectionEvent;public class FontsListJFrame extends JFrame implements ListSelectionListener{    private JList list;    private JTextArea text;        public FontsListJFrame()    {        super("系统字体预览");        Dimension dim = getToolkit().getScreenSize();      //获得屏幕分辨率        this.setBounds(dim.width/4,dim.height/4,dim.width/2,dim.height/2);//窗口居中        this.setDefaultCloseOperation(EXIT_ON_CLOSE);        GraphicsEnvironment ge=GraphicsEnvironment.getLocalGraphicsEnvironment();        String[] fontsName=ge.getAvailableFontFamilyNames(); //获得系统字体        this.list = new JList(fontsName);              this.getContentPane().add(new JScrollPane(this.list));        this.list.addListSelectionListener(this);          //注册选择事件监听器        this.list.setCellRenderer(new CheckBoxListCellRenderer()); //设置自定义的单元渲染器                this.text = new JTextArea("Welcome  欢迎");        this.getContentPane().add(this.text,"South");        this.setVisible(true);    }    class CheckBoxListCellRenderer extends JCheckBox implements ListCellRenderer  //列表框单元渲染器    {                                                      //必须是内部类        public Component getListCellRendererComponent(JList list,Object value,int index,boolean isSelected,boolean cellHasFocus)        {            setText(value.toString());            setFont(new Font(value.toString(),Font.BOLD,16));       //以当前字符串作为字体名设置字体            setBackground(isSelected ? Color.lightGray : Color.white); //选中数据项背景浅灰色            setForeground(isSelected ? Color.red : Color.black);    //选中数据项红色显示            setSelected(isSelected);                                //当前数据项选中状态            return this;        }    }    public void valueChanged(ListSelectionEvent e)         //在列表框中选择数据项时触发    {                                        //一次选择，执行两次该事件？？为什么        String selected = (String)list.getSelectedValue();//        System.out.println(selected);        this.text.setFont(new Font(selected,Font.BOLD,56));    }        public static void main(String[] args)     {        new FontsListJFrame();    }}

        this.list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);   //设置为单选  ////////////////////////////////        if (e.getValueIsAdjusting()) {            String selected = (String) list.getSelectedValue();//            System.out.println(selected);            this.text.setFont(new Font(selected, Font.BOLD, 56));        }