Here is a code that generates id into database. In the given code, we have fetched the last id from the database table and by adding 1, we have generated the next id and inserted into database table.

import java.sql.*;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class ComboBox{

    public static void main(String[] args) throws Exception{
        JFrame f=new JFrame();
        f.setLayout(null);
        JLabel lab=new JLabel("Select Name:");

        final String st[] = {"Add Employee","Edit Employee","Delete Employee","View Employee"};
        final JComboBox combo=new JComboBox(st);
        JButton b=new JButton("Submit");

        b.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent actionEvent) {
        Object obj = combo.getSelectedItem();
        String sel=obj.toString();
        System.out.println(sel);
        if(sel.equals(st[0])){
        try{
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "root" );
        Statement st=conn.createStatement();
        Statement stmt=conn.createStatement();
        ResultSet rs=st.executeQuery("select * from data1");
        int id=0;
        while(rs.next()){
            id=rs.getInt("id");
        }
        System.out.println(id);
        int ide=id+1;
        stmt.executeUpdate("insert into data1(id) values("+ide+")");
        System.out.println("Inserted");
        }
        catch(Exception e){
        System.out.println(e);
        }
        }
        }
       });
       lab.setBounds(20,20,100,20);
       combo.setBounds(120,20,80,20);
       b.setBounds(220,20,80,20);
       f.add(lab);
       f.add(combo);
       f.add(b);
       f.setVisible(true);
       f.setSize(300,120);
    }
}