Reason for "AWT-EventQueue-0" java.lang.NullPointerException error

Question

I'm creating a video game in Java and I am trying to retrieve a list of coordinate points and draw them onto the map, but every time I run my application I receive a "AWT-EventQueue-0" java.lang.NullPointerException and have not been able to figure out why it's pointing to null. Here is the error report I receive:

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at game.PathPoints.drawObjectPath(PathPoints.java:23)
at game.TestPanel.paintComponent(TestPanel.java:64)
at javax.swing.JComponent.paint(JComponent.java:1029)
at javax.swing.JComponent.paintChildren(JComponent.java:866)
at javax.swing.JComponent.paint(JComponent.java:1038)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:581)
at javax.swing.JComponent.paintChildren(JComponent.java:866)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5145)
at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:302)
at javax.swing.RepaintManager.paint(RepaintManager.java:1216)
at javax.swing.JComponent.paint(JComponent.java:1015)
at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:78)
at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:115)
at java.awt.Container.paint(Container.java:1784)
at javax.swing.RepaintManager$3.run(RepaintManager.java:818)
at javax.swing.RepaintManager$3.run(RepaintManager.java:795)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:795)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:764)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:706)
at javax.swing.RepaintManager.access$1000(RepaintManager.java:61)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1621)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:226)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:673)
at java.awt.EventQueue.access$300(EventQueue.java:96)
at java.awt.EventQueue$2.run(EventQueue.java:634)
at java.awt.EventQueue$2.run(EventQueue.java:632)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:643)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:275)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:200)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:185)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:177)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:138)

The code of the error for the class PathPoints is here:

    package game;

    import java.awt.Color;
    import java.awt.Graphics;  
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Scanner;

    public class PathPoints {

        public PathPoints (Scanner s){
            int numberCoord;
            List<Coordinates> path = new ArrayList<Coordinates>();
            numberCoord = s.nextInt();
            for(int x = 0; x < numberCoord; x++){
               Coordinates points = new Coordinates(s.nextInt(), s.nextInt());
               path.add(points);
        }

        }
        public void drawObjectPath(Graphics g){
            g.setColor(Color.GREEN);
            for(int n = 0; n < path.size()- 1; n++){
                g.drawLine(((Coordinates)path.get(n)).x, ((Coordinates)path.get(n)).y, ((Coordinates)path.get(n+1)).x, ((Coordinates)path.get(n+1)).y);
                g.drawLine(((Coordinates)path.get(n)).x, ((Coordinates)path.get(n)).y+1, ((Coordinates)path.get(n+1)).x, ((Coordinates)path.get(n+1)).y+1);

    }
}

List path;
}    

The code for the class TestPanel is here:

    public class TestPanel extends JPanel implements MouseListener
{
    private static final long serialVersionUID = 1L;  // Ignore this - It's just to get rid of a warning.

    // Instance variable(s).
    int x,y;
    private Image backdrop;
    PathPoints objectPath;

    /**
    * Constructor - loads a background image
    */
    public TestPanel ()
    {
        try
        {
            ClassLoader myLoader = this.getClass().getClassLoader();
            InputStream imageStream = myLoader.getResourceAsStream("resources/path_1.jpg");
            backdrop = ImageIO.read(imageStream);
            InputStream pointStream = myLoader.getResourceAsStream("resources/path_1.txt");
            Scanner s = new Scanner (pointStream);
            objectPath = new PathPoints(s);



        }
        catch (IOException e)
        {
            System.out.println ("Could not load: " + e);
        }
        addMouseListener(this);
    }



    /**
    * This paint meethod draws the background image anchored
    * in the upper-left corner of the panel.  
    */
    public void paintComponent (Graphics g)
    {
        g.drawImage(backdrop, 0, 0, null);
        objectPath.drawObjectPath(g);
        //Coordinates();
    }

I have tested the constructor of PathPoints to see if it is correctly adding the values to the ArrayList and it is adding them correctly, so what part should I look at next to solve this?

Solution 1

public PathPoints (Scanner s){
  int numberCoord;
  List<Coordinates> path = new ArrayList<Coordinates>();
  numberCoord = s.nextInt();
  for(int x = 0; x < numberCoord; x++){
    Coordinates points = new Coordinates(s.nextInt(), s.nextInt());
    path.add(points);
  }

In those lines, "path" does not refer to the attribute of your class but to a locale variable. Hence your attribute "path" is never initialized.

btw, 2 recommandations :

• Never name a variable with the name of one of your attributes.

• Your attributes should be private:

  private List path;

Solution 2

You are shadowing the variable path. A local variable with the same name is instantiated in the scope of the constructor PathPoints but the class member variable path is null in the method drawObjectPath. Replace

List<Coordinates> path = new ArrayList<Coordinates>();

with

path = new ArrayList<Coordinates>();

Solution 3

In the PathPoints class constructor you are hiding the member variable path with a local variable:

        int numberCoord;
        List<Coordinates> path = new ArrayList<Coordinates>();
        numberCoord = s.nextInt();

This means that the member variable is never assigned a value and will be null when the drawObjectPath method is called.

To fix this, don't hide the member variable; use:

        int numberCoord;
        path = new ArrayList<Coordinates>();
        numberCoord = s.nextInt();