Java6学习笔记7——利用Swing开发下拉列表

import javax.swing.JFrame;
import javax.swing.JComboBox;
import java.awt.Container;
import java.awt.Insets;
import java.awt.Dimension;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.event.WindowEvent;
import java.awt.event.WindowAdapter;

public class ComboDemo extends JFrame implements ActionListener {
JComboBox combo;
String planets[] = { "Mercury","Venus","Earth","Mars",
"Jupiter","Saturn","Uranus","Neptune","Pluto" };
public static void main(String arg[]) {
new ComboDemo();
}
public ComboDemo() {
int height;
int width;
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
} );
setLocation(250,150);
Container pane = getContentPane();
pane.setLayout(null);
Insets insets = pane.getInsets();

combo = new JComboBox(planets);//创建下拉列表的同时把内容也填进去了。
combo.setSelectedIndex(2);//设置默认索引为2
combo.addActionListener(this);
Dimension dim = combo.getPreferredSize();
int x = insets.left + 20;
int y = insets.top + 20;
combo.setBounds(x,y,dim.width,dim.height);
pane.add(combo);

width = x + dim.width + 20 + insets.left;
height = y + dim.height + 40 + insets.bottom;

setSize(width,height);
setVisible(true);
}
public void actionPerformed(ActionEvent e) {
String str = (String)combo.getSelectedItem();//获取所选的内容
System.out.println(str);
int index = combo.getSelectedIndex();
System.out.println("index=" + index);//获取所选的索引
}
}

注：在这个例子中，若是想让此下拉列表可以允许用户自己填入一个选项的话，使用combo.setEditable(true);用户可以自行填入一个选项，返回的时候这个选项的索引为-1，用户自定义选项不影响其余选项的内容和索引。