ListSelectionListener响应两次

[wd9053]

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-1

import 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();
	}
}

[joaryyu]

因为你鼠标按下和释放都会触发事件。

[joaryyu]

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();
    }
}