Minh k tim ra loi de sua mong cac ban giup minh .....hix...thanks cac ban nhieu(if tim ra chi minh cac sua lun nhe..hihi)
import java.awt.Component;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import javax.swing.ImageIcon;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.border.LineBorder;
import java.awt.Point.*;
public class test1 implements ItemListener {
private JFrame frame;
private JComboBox cb;
private JLabel display;
private final String[] pet = {"2","4","5","7","10","13","14","15",
"16","17","18","19","20","21","22","23"};
private final String[] images = {"2.jpeg","4.jpeg","5.jpeg","7.jpeg","10.jpeg","13.jpeg",
"14.jpeg","15.jpeg","16.jpeg","17.jpeg","18.jpeg","19.jpeg","20.jpeg",
"21.jpeg","22.jpeg","23.jpeg"};
ImageIcon icon = new ImageIcon(ClassLoader.getSystemResource("images//2.jpeg"));
public test1() {
frame = new JFrame();
cb = new JComboBox(pet);
cb.setSelectedIndex(0);
cb.setPreferredSize(new Dimension(140, 22));
cb.addItemListener(this);
//set frame
frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE);
frame.setLayout(new FlowLayout());
frame.setSize(new Dimension(450, 250));
frame.setTitle ("JComboBox example");
frame.add(cb);
//set up label
display = new JLabel();
display.setPreferredSize(new Dimension(250, 200));
display.setMaximumSize(new Dimension(100, 127));
display.setAlignmentX(Component.CENTER_ALIGNMENT);
display.setAlignmentY(Component.CENTER_ALIGNMENT);

display.setBorder(LineBorder.createGrayLineBorder());
frame.add(display);
frame.setVisible(true);
}
@Override
public void itemStateChanged(ItemEvent e) {
if (e.getStateChange() == ItemEvent.SELECTED) {
cb = ( JComboBox) e.getSource();
int index = cb.getSelectedIndex();
display.setIcon(new ImageIcon(ClassLoader.getSystemResource ("images//" + images[index])));
frame.setTitle (icon.getIconHeight() + " " + icon.getIconWidth());
}
}
public static void main(String[] args) {
test1 a = new test1();
}
}

dùng cái label rỗng, hoặc button rỗng, textfield rỗng.... rồi set icon cho nó!!!