Question

请教java组合框的事件监听器

public class SearchEngine implements ItemListener{
JComboBox jcb = new JComboBox();
String s="";
public SearchEngine(){
jcb.addItem("");
jcb.addItem("China");
jcb.addItem("France");
jcb.addItem("Russia");
jcb.addItemListener(this);
//其余代码未写上
}

public void itemStateChanged(ItemEvent e){
s = jcb.getSelectedItem().toString();
System.out.println(s);
}
}

对于这个代码，我希望每当选择了组合框的一项后，屏幕会打印一次该选项，但为什么当我选择了组合框的一项后，屏幕总是打印两遍项目的内容呢？
请各位帮忙指教一下
监听器没有注册两次啊！

Answer 1

根据文档上说的
Combo boxes also generate item events, which are fired when any of the items' selection state changes. Only one item at a time can be selected in a combo box, so when the user makes a new selection the previously selected item becomes unselected.Thus two item events are fired each time the user selects a different item from the menu.

因此实际上一个是unselected消息，一个是selected消息，所以有两次

lz如果想获取新的选项信息，应该使用ActionListener,而不是ItemListener
以下是文档上的例子

public class ComboBoxDemo ... implements ActionListener {
. . .
petList.addActionListener(this) {
. . .
public void actionPerformed(ActionEvent e) {
JComboBox cb = (JComboBox)e.getSource();
String petName = (String)cb.getSelectedItem();
updateLabel(petName);
}
. . .
}