ListSelectionListener响应两次

Question

JList中添加了ListSelectionListener，每次打印出当前所选的索引，可是为什么每次鼠标点击后会ListSelectionListener会响应两次？我一开始以为是先将当前所选项置为空，然后将当前所选项置为目标值，但是从打印结果来看，centerList.getSelectedIndex（）并未返回-1import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
public class Test extends JFrame
{
JList centerList = new JList();
public Test()
{
String[] s = {"a", "b", "c"};
centerList.setListData(s);
centerList.addListSelectionListener(new ListSelectionListener()
{
  public void valueChanged(ListSelectionEvent e)
  {
      System.out.println(centerList.getSelectedIndex());
  }
});

add(centerList);

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setPreferredSize(new Dimension(400, 400));
pack();
setVisible(true);
}

public static void main(String[] args)
{
new Test();
}
}

Answer 1

这样的话，你要是只打执行一次打印协作。
下面可以。import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import java.awt.event.MouseEvent;public class JFrameDemo extends JFrame
{
    JList centerList = new JList();
    boolean flag = false; //鼠标是否按下的标志
    public JFrameDemo()
    {
        String[] s = {"a", "b", "c"};
        centerList.setListData(s);
        centerList.addListSelectionListener(new ListSelectionListener()
        {
          public void valueChanged(ListSelectionEvent e)
          {
           if(flag == true) {//仅当鼠标按下时执行打印动作
            System.out.println(centerList.getSelectedIndex());
            flag = false;
           }
          }
        });
        
        centerList.addMouseListener(new MouseAdapter() {
         public void mousePressed (MouseEvent e) {
         flag = true;
         }
        });
        
        add(centerList);
        
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(400, 400));
        pack();
        setVisible(true);
    }
    
    public static void main(String[] args)
    {
        new JFrameDemo();
    }
}