Hi Friend,

Try this,

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

class JComboBoxEx extends JFrame
{
    JComboBoxEx(){
        setLayout(null);
    JLabel lab1=new JLabel("Select");
    final JComboBox combo=new JComboBox();
    combo.addItem("--Select--");
    for(int i=1;i<=10;i++){
    combo.addItem(i);
    }
    JLabel lab2=new JLabel("Factorial");
    final JTextField text=new JTextField(20);
    combo.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
               JComboBox comboBox = (JComboBox) event.getSource();
               String st= combo.getSelectedItem().toString();
               int m=Integer.parseInt(st);
               long num=m;
               for(int i=m;i>1;i--){
               num=num*(i-1);
               }
               text.setText(Long.toString(num));
            }
            });
            lab1.setBounds(10,10,100,20);
            combo.setBounds(150,10,100,20);
            lab2.setBounds(10,40,100,20);
            text.setBounds(150,40,100,20);
            add(lab1);
            add(combo);
            add(lab2);
            add(text);
            setVisible(true);
            setSize(300,100);
    }
    public static void main(String[] args) 
    {
        new JComboBoxEx();
    }
}

Thanks