"Unknown Source" errors

Question

The application behind this code is generating a bunch of "Unknown Source" errors at runtime:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Polygon#MouseHandler.mousePressed(Polygons.java:95)
at java.awt.Component.processMouseEvent(Unknown Source), and so on...
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.ArrayList;
public class Polygons extends JPanel {
        protected static boolean shiftIsDown;
        public static void main(String[] args) {
                JFrame window = new JFrame("Polygons");
                Polygons content = new Polygons();
                content.addKeyListener( new KeyAdapter() {
                                        public void KeyPressed(KeyEvent evt) {
                                                int key = evt.getKeyCode();
                                                if(key == KeyEvent.VK_SHIFT)
                                                        shiftIsDown = true;
                window.setContentPane(content);
                window.pack();
                window.setLocation(100, 100);
                window.setResizable(false);
                window.setDefaultCloseOperation( JFrame.DISPOSE_ON_CLOSE );
                window.setVisible(true);
        } // End main()
        private static class PolygonData {
                int[] x;
                int[] y;
        public Polygons() {
        // Constructor.
                setBackground(Color.WHITE);
                MouseHandler listener = new MouseHandler();
                setPreferredSize(new Dimension(350, 350) );
                addMouseListener(listener);
                addMouseMotionListener(listener);
        private class MouseHandler implements MouseListener,
                MouseMotionListener {
                PolygonData polygon;
                private boolean dragging = false;
                private int vertices;
                private int x1, y1;   // First vertex point.
                private int x2, y2;   // Second vertex point.
                private boolean newPolygon = false; // Is the user drawing
                                                    // a new polygon?
                       public void mousePressed(MouseEvent evt) {
                                // If the user is within two pixels of the
                                // starting point, draw the polygon.
                                // Graphics g = getGraphics();
                                int tolerence = 2;
                                if(shiftIsDown);
                                x1 = evt.getX();
                                y1 = evt.getY();
                                vertices++;
                                if(newPolygon) {
                                        polygon = new PolygonData();
                                        x2 = x1;
                                        y2 = y1;
                                        polygon.x = new int[1024];
                                        polygon.y = new int[1024];
                                if(vertices == 1){
                                        polygon.x = new int[1024];
                                        polygon.y = new int[1024];
                                polygon.x[vertices] = x1;
                                polygon.y[vertices] = y1;
                                g.setColor(Color.BLACK);
                                g.drawPolygon(polygon.x, polygon.y, vertices);
                                // Did the user click near the starting point?
                                if( (polygon.x[0] - x1 <= tolerence)
                                        && (polygon.y[0] - y1 <= tolerence) ) {
                                                g.setColor(Color.RED);
                                                g.fillPolygon(polygon.x, polygon.y, vertices);                                }                                               
                       public void mouseMoved(MouseEvent evt) {
                                // Grab the graphics context.
                                Graphics g = getGraphics();
                                x2 = evt.getX();
                                y2 = evt.getY();
                                g.drawLine(x1, y1, x2, y2);
                                repaint();
                       // Required by the interfaces....
                       public void mouseReleased(MouseEvent evt) { }
                       public void mouseClicked(MouseEvent evt) { }
                       public void mouseEntered(MouseEvent evt) { }
                       public void mouseExited(MouseEvent evt) { }
                       public void mouseDragged(MouseEvent evt) { }
        } // End nested class Mousehandler
} // End class PolygonsSame goes for mouseMoved().... What's the fix?

Answer 1

flounder wrote:
if(newPolygon) {
polygon = new PolygonData();This is the probable cause. Since newPolygon is initially false, it will not execute the if statement and polygon will not be initialised. So later in your code when you attempt to use polygon it is null.That did it.
Thanks!