JTabbedPane not maximized in frame.

Question

Hi,
I have an swing application which comprises of this code:
(the code hash as been shortened). My problem is when the user clicks on
the button from LoginPanel, the panel gets removed and get replaced with a JTabbedPane(AgentMain). However, the JTabbedPane is not correctly displayed, meaning it is not fully maximized to the frame. It is just centered in the middle of the frame with a set size. How can I make the JTabbedPane fully maximized with the frame?
1. class REASApp extends JFrame{
... etc
getContentPane().add(BorderLayout.CENTER,new LoginPanel());
public static void main(String[] args){          
     ReasApp mp = new ReasApp();
     mp.pack();
     mp.setSize(600, 400);
     mp.setBackground(Color.white);
     mp.setVisible(true);
} // Closes main method
2. class LoginPanel extends JPanel{
JPanel mainPanel = new JPanel();
add(mainPanel);
public void actionPerformed(ActionEvent evt){
          if (evt.getSource() == bLogin){
          removeAll();
          add(new AgentMain());
          revalidate();
          repaint();
3. class AgentMain extends JTabbedPane{
... etc
}

Answer 1

change the layout
setLayout(new BorderLayout());