Retrieve values from database and stored it into JComboBox:

import java.sql.*;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class  JComboBoxExample {
    public static void main(String[] args) 
    {
        JFrame f=new JFrame();
        f.setLayout(null);
        JLabel lab=new JLabel("Course Items:");
        final JComboBox combo=new JComboBox();
        combo.addItem("--Select--");
        try{
        Class.forName("com.mysql.jdbc.Driver");
           Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "root");
           Statement st=con.createStatement();
           ResultSet rs=st.executeQuery("select * from student");
           while(rs.next()){
           combo.addItem(rs.getString("course_items"));
           }
        }
        catch(Exception e){}
        JButton b=new JButton("Get");
        lab.setBounds(20,20,100,20);
        combo.setBounds(120,20,150,20);
        b.setBounds(120,50,80,20);
        b.addActionListener(new ActionListener(){
         public void actionPerformed(ActionEvent e){
        String value = combo.getSelectedItem().toString();

        JOptionPane.showMessageDialog(null,"You have selected '"+value+"' from ComboBox");
         }
         });
        f.add(lab);
        f.add(combo);
        f.add(b);
        f.setVisible(true);
        f.setSize(300,120);
    }
}