java 中 JComboBox事件有有关问题

 
 import javax.swing.*; 
 import javax.swing.border.TitledBorder; 
 import java.awt.event.*; 
  
 import java.awt.*; 
 import java.io.*; 
  
 import java.util.Enumeration; 
  
 public class css extends JFrame{ 
 	int J=0; 
 	JPanel p1 = new JPanel(); 
 	 
 	private JLabel l2 = new JLabel("Com:"); 
 	private JComboBox bo2 = new JComboBox();	 
 	public css(){		 
 		 
 		this.getContentPane().setLayout(null); 
 		p1.setBounds(0, 0, 170, 186); 
 		p1.setLayout(null);		 
 		p1.setBorder(new TitledBorder("Settings"));		 
 		l2.setBounds(5, 20, 60, 20); 
 		bo2.setBounds(50,20, 110, 20);				 
 		p1.add(l2); 
 		p1.add(bo2);	 
 		this.getContentPane().add(p1);		 
 		this.getPort();		 
 		bo2.addItemListener(new ItemListener(){ 
  
 			public void itemStateChanged(ItemEvent e) { 
 				 
 				boC_change(e);				 
 			} 
 			 
 		}); 
 		 
 		bo2.setSelectedIndex(0);		 
 		this.setSize(340,310); 
 		this.setVisible(true); 
 	}	 
 	 
 	public void boC_change(ItemEvent e){ 
 	 
 		System.out.println(bo2.getSelectedItem().toString()); //用鼠标改变bo2的值的时候，会发现输出两行..这是什么,,我只点了一次 
 		 
 	} 
 	 
  
  
     public void getPort(){ 
 		 
  
 		bo2.addItem("select"); 
 		bo2.addItem("selec1t"); 
  
  
 	 
 }	public static void main(String args[]){ 
 		 
 		 
 		 
 		css m = new css(); 
 		ImageIcon icon=new ImageIcon("sure.gif"); 
 		m.validate();		 
 		Image image=icon.getImage(); 
 		m.setIconImage(image); 
 		m.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); 
  
 		 
 	} 
 	 
 	 
  
 }