Java SE (Java Platform, Standard Edition)

Here when i press show button JInternalframe contains JTable appear . But i want JinternalFrame to full the screen so i used :

        try {
            internalFrame.setMaximum(true);
        } catch (PropertyVetoException ex) {
            ex.printStackTrace();
        }

but i t give me : Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.DefaultDesktopManager.maximizeFrame(DefaultDesktopManager.java:115)

code:
--------------

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.beans.PropertyVetoException;
import java.sql.SQLException;
import javax.swing.JButton;
import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JInternalFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.ScrollPaneConstants;

public class TestJtable extends JFrame {

    private JPanel mainPanel, buttonPanel;
    private JButton[] buttons;
    private final String[] BUTTONNAMES = {"showButton", "insertButton", "searchButton", "updateButton", "deletebutton"};
    private JInternalFrame internalFrame;
    private JDesktopPane desktopPane;
    private JTable table;
    private JScrollPane scrollPane;
    private MyActionListener myActionListener;

    public TestJtable() throws ClassNotFoundException, SQLException {
        intializingPanels();
        intializingbuttons();
        table = new JTable(3,4);
        scrollPane = new JScrollPane(table, ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS,
                ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
        getContentPane().add(mainPanel);
    }

    public static void main(String[] args) throws SQLException, ClassNotFoundException {
        TestJtable testJtable = new TestJtable();
        testJtable.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        testJtable.setSize(new Dimension(600, 300));
        testJtable.setLocationRelativeTo(null);
        testJtable.setVisible(true);
        testJtable.setExtendedState(MAXIMIZED_BOTH);
    }

    private void intializingPanels() {
        mainPanel = new JPanel(new BorderLayout());
        buttonPanel = new JPanel(new FlowLayout());

    }

    private void intializingbuttons() {
        myActionListener = new MyActionListener();
        buttons = new JButton[BUTTONNAMES.length];
        for (int i = 0; i < BUTTONNAMES.length; i++) {
            buttons[i] = new JButton(BUTTONNAMES);

buttons[i].addActionListener(myActionListener);

buttonPanel.add(buttons[i]);

mainPanel.add(buttonPanel, BorderLayout.NORTH);

}

}



private void intializingInternalDeskTop() {

JPanel panel = new JPanel(new BorderLayout());

panel.add(scrollPane, BorderLayout.CENTER);

/////////////////////////////////////////////////////////////

internalFrame = new JInternalFrame("", true, true, true);

internalFrame.setSize(500, 500);

internalFrame.add(panel);

internalFrame.setVisible(true);

try {

internalFrame.setMaximum(true);

} catch (PropertyVetoException ex) {

ex.printStackTrace();

}

//////////////////////////////////////////////////////////////

desktopPane = new JDesktopPane();

desktopPane.setOpaque(false);

desktopPane.setEnabled(true);

desktopPane.add(internalFrame);

mainPanel.add(desktopPane, BorderLayout.CENTER);

}



private class MyActionListener implements ActionListener {



public void actionPerformed(ActionEvent e) {

if (e.getSource() == buttons[0]) {

intializingInternalDeskTop();



}

}

}

}
Thanks in advance