Calling a drawLine() from one class to another from an ActionEvent

Question

I have several JPanel objects called and placed on a JFrame. The JFrame has a RadioButton group with radio buttons. If I select a radio button and call a drawLine() method from a JPanel, I receive a "NullPointerException". Is it not possible to call this graphic method from one class to another?
Thanks for any input you can provide.
John

Answer 1

Remember that each panel draws it's own current state. So you need the ActionPerformed to change the state in your target panel.
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.event.*;
public class PanelComm extends JPanel {
  private SubPanelOne subPanelOne = new SubPanelOne();
  private SubPanelTwo subPanelTwo = new SubPanelTwo();
  public class SubPanelOne extends JPanel {
    public SubPanelOne () {
      setLayout ( new BorderLayout() );
      setBorder ( BorderFactory.createTitledBorder ( "SubPanel One" ) );
      Reactor     reactor    = new Reactor();
      ButtonGroup group      = new ButtonGroup();
      JPanel      radioPanel = new JPanel(new GridLayout(0, 1));
      JRadioButton buttonOne = new JRadioButton ( "One" );
      buttonOne.addActionListener ( reactor );
      group.add ( buttonOne );
      radioPanel.add ( buttonOne );
      JRadioButton buttonTwo = new JRadioButton ( "Two" );
      buttonTwo.addActionListener ( reactor );
      group.add ( buttonTwo );
      radioPanel.add ( buttonTwo );
      JRadioButton buttonThree = new JRadioButton ( "Three" );
      buttonThree.addActionListener ( reactor );
      group.add ( buttonThree );
      radioPanel.add ( buttonThree );
      add ( radioPanel ,BorderLayout.LINE_START );
    protected void paintComponent ( Graphics _g ) {
      super.paintComponent ( _g );
      Graphics2D g = (Graphics2D)_g;
      g.setRenderingHint ( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );
  public class SubPanelTwo extends JPanel {
    private JLabel text = new JLabel();
    public SubPanelTwo () {
      setLayout ( new BorderLayout() );
      setBorder ( BorderFactory.createTitledBorder ( "SubPanel Two" ) );
      text.setFont ( new Font ( "Verdana" ,Font.PLAIN ,30 ) );
      text.setHorizontalAlignment ( JLabel.CENTER );
      add ( text ,BorderLayout.CENTER );
    protected void paintComponent ( Graphics _g ) {
      super.paintComponent ( _g );
      Graphics2D g = (Graphics2D)_g;
      g.setRenderingHint ( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );
    public void setChoice ( String cmd ) {
      text.setText ( cmd );
  public class Reactor implements ActionListener {
    public void actionPerformed ( ActionEvent e ) {
      subPanelTwo.setChoice ( e.getActionCommand() );
  public PanelComm () {
    setLayout ( new GridLayout ( 1 ,2 ) );
    add ( subPanelOne );
    add ( subPanelTwo );
  //  main
  public static void main ( String[] args ) {
    JFrame f = new JFrame ( "Panel Communication" );
    f.setDefaultCloseOperation ( JFrame.EXIT_ON_CLOSE );
    f.getContentPane().add ( new PanelComm() ,BorderLayout.CENTER );
    f.setSize ( 320 ,120 );
    f.setVisible ( true );
  }  // main
}