Here is a Login application that checks whether the username and password is valid or not. If the user is valid, it will continue to the next page. Otherwise shows an error message.

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

class LoginDemo extends JFrame{
 JButton SUBMIT;
 JLabel label1,label2,lab;
 final JTextField  text1,text2;
  LoginDemo(){

    setTitle("Login Form");
    setLayout(null);
    label1 = new JLabel();
    label1.setText("Username:");
    text1 = new JTextField(15);

    label2 = new JLabel();
    label2.setText("Password:");
    text2 = new JPasswordField(15);

    SUBMIT=new JButton("SUBMIT");
    label1.setBounds(350,100,100,20);
    text1.setBounds(450,100,200,20);
    label2.setBounds(350,130,100,20);
    text2.setBounds(450,130,200,20);
    SUBMIT.setBounds(450,160,100,20);
    add(label1);
    add(text1);
    add(label2);
    add(text2);
    add(SUBMIT);

   setVisible(true);
   setSize(1024,768);

 SUBMIT.addActionListener(new ActionListener(){
   public void actionPerformed(ActionEvent ae){
    String value1=text1.getText();
    String value2=text2.getText();
    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 login where username='"+value1+"' and password='"+value2+"'");
           String uname="",pass="";
           if(rs.next()){
               uname=rs.getString("username");
               pass=rs.getString("password");
           }
 if(value1.equals("") && value2.equals("")) {
      JOptionPane.showMessageDialog(null,"Enter login name or password","Error",JOptionPane.ERROR_MESSAGE);
  }
 else if(value1.equals(uname) && value2.equals(pass)) {
    NextPage page=new NextPage(uname);
    page.setVisible(true);
    }
 else if (!value1.equals(uname) && !value2.equals(pass)) {
     text1.setText("");
     text2.setText("");
    }
    }
    catch(Exception e){}
}
 });
  }

  public static void main(String arg[]){
  new LoginDemo();
}
}


class NextPage extends JFrame
{
  NextPage(String st)
   {
      setLayout(null);
     setDefaultCloseOperation(javax.swing. WindowConstants.DISPOSE_ON_CLOSE);
     setTitle("Welcome");
     JLabel lab=new JLabel("Welcome  "+st);

     lab.setBounds(10,10,500,20);
     add(lab);

       setSize(1024, 768);
      }
 }