You should avoid using null layout or absolute positioning for arranging swing components. Always use the best appropriate layout manager in the situation since it has a lot of advantages. The best layout to handle your current situation is GridLayout
Here is the modified version of your code using GridLayout
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JToggleButton;
import javax.swing.border.EmptyBorder;
public class Book_GUI extends JFrame {
// private EconomyClass eco;
private JPanel contentPane;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
Book_GUI frame = new Book_GUI();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public Book_GUI() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// setBounds(100, 100, 450, 300);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
contentPane.setLayout(new GridLayout(9, 5));
// contentPane.setLayout(new BorderLayout(0, 0));
setContentPane(contentPane);
// this.add(contentPane);
JButton btnBookFlight;
// eco = new EconomyClass();
// eco.setSeats(5);
for (int i = 0; i < 45; i++) {
btnBookFlight = new JButton("Book" + i);
btnBookFlight.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
JButton button = (JButton) arg0.getSource();
;
button.setBackground(Color.RED);
button.setOpaque(true);
}
});
// btnBookFlight.setBounds(77, 351, 100, 23);
contentPane.add(btnBookFlight);
}
pack();
}
}
Further read : A Visual Guide to Layout Managers