java null pointer and paint dirty regions error

**Spidey1980** · August 16th, 2011, 03:38 PM

I have a project for class. It compiles but does not run and I just cannot understand why.

It is a not so simple starscape.

my folder set up is StarScape/Star/

I have my main applet in StarScape, and a package called star in star.

This is the command I use to compile it:
C:\JSource\StarScape>javac StarScape.java Star/Star.java

code for StarScape.java in the starscape folder:

// starscape.java
// by Jon Crawford
import Star.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.geom.*;
 
public class StarScape extends JApplet 
{
  final int maxStars = 100;
  String key = "";
  int index;
  int myX;
  int myY;
  int myRadius;
  int myLayer;
  int rectX;
  int rectY;
  int rectXX;
  int rectYY;
 
  public void paint(Graphics g)
  {
    Star stars[] = new Star[maxStars];//make 100 star objects
    g.setColor(Color.black);
    g.fillRect(0,0,600,600);// make the background black
    do
    {
      for (index = 0; index < maxStars; index++) 
      {
        stars[index].getStarX(myX);
        stars[index].getStarY(myY);
        stars[index].getStarRadius(myRadius);//get data for star at index number
        rectX = myX - myRadius + 399;//myX and myY are based on a 0,0 in the center grid
        rectY = myY - myRadius + 299;//as well as converting to a 0,0 at top left grid
        rectXX = myX + myRadius + 399;//these lines setup the boundaries of
        rectYY = myY + myRadius + 299;//a rectangular star
        g.setColor(Color.black);
        g.fillRect(rectX, rectY, rectXX, rectYY);//erase old star
        stars[index].moveStar();//move the star
        stars[index].getStarX(myX);
        stars[index].getStarY(myY);
        stars[index].getStarRadius(myRadius);//get data agian because star was moved
        stars[index].getStarLayer(myLayer);//layer just denotes shade of star. 
        if (myLayer == 1) 
        {
          g.setColor(Color.white);
        }else if (myLayer == 2) 
        {
          g.setColor(Color.lightGray);
        }else if (myLayer == 3) 
        {
          g.setColor(Color.darkGray);
        }else 
        {
          g.setColor(Color.white);//these set color based on layer
        }
        rectX = myX - myRadius + 399;
        rectY = myY - myRadius + 299;
        rectXX = myX + myRadius + 399;
        rectYY = myY + myRadius + 299;//set up the new star I mean rectangle
        g.fillRect(rectX, rectY, rectXX, rectYY);//draw star
      }//end for
    }while (key == "");//should implement key listener
  }//end paint
}//end starscape

Code for star.java in the star folder:

// star.java
// by Jon Crawford
package Star;
 
import java.util.*;
 
public class Star {
 
  private final int starSpeed = 4;
  private final int starRadius = 1;
  private final int starSteps = 0;
  private final int[] invalidValues = new int[]{ -399, -299, 399, 299 };
  private int xPos;
  private int yPos;
  private int starAngle;
  private int starLayer;
  private int starField[] = new int[8];
  Random rand = new Random();
 
  Star(){
    xPos = rand.nextInt(399) - 199;//star starts in a small square
    yPos = rand.nextInt(299) - 149;//in the center
    starAngle = (int)(Math.sqrt(xPos * xPos)+(yPos * yPos));// calculate the hypotenuse   
    starLayer = rand.nextInt(3);//random layer
    starField[0] = xPos;//set data to star's arrary
    starField[1] = yPos;//this is for current position
    starField[2] = xPos / starAngle * starSpeed;//these set the next position
    starField[3] = yPos / starAngle * starSpeed;
    starField[4] = starLayer;//back, mid, or foreground layer
    starField[5] = starRadius;//star starting size
    starField[6] = starSpeed;//the starting speed
    starField[7] = starSteps;//star has taken 0 steps, every 5 steps star grows and speeds up
    starField[8] = starAngle;//store the hypotenuse
  }//end default constructor
 
  public void moveStar() 
  {//set or mutator method
    if(starField[2] > invalidValues[0] && starField[3] > invalidValues[1] && starField[2] < 
 
invalidValues[1] && starField[3] < invalidValues[3])
    {//if star is not to close to the edge
      starField[0] = starField[2];//next position becomes the curent position 
      starField[1] = starField[3];       
      starField[2] = starField[0] / starField[8] * starField[6];//new next position
      starField[3] = starField[1] / starField[8] * starField[6];
      starField[7]++;//increment steps
      if (starField[7] / 5 == (int)(starField[7] / 5) && starField[5] < 15)
      {//if star has taken 5 steps and is not larger than 30x30
        starField[5]++;//increment size
        starField[6]++;//increment speed
      }//end if      
    }else
    {//if star is too close to the edge
      xPos = rand.nextInt(399) - 199;//set new starting position
      yPos = rand.nextInt(299) - 149;
      starAngle = (int)(Math.sqrt(xPos * xPos)+(yPos * yPos));//calculate new hypotenuse
      starLayer = rand.nextInt(3);//new random layer
      starField[0] = xPos;//set data to star's array
      starField[1] = yPos;//same as default constructor
      starField[2] = xPos / starAngle * starSpeed;
      starField[3] = yPos / starAngle * starSpeed;
      starField[4] = starLayer;
      starField[5] = starRadius;
      starField[6] = starSpeed;
      starField[7] = starSteps;
      starField[8] = starAngle;
    }//end if
  }//end moveStar
 
  public int getStarX(int myX) 
  {//get method
    return starField[0];
  }//end getStarX
 
  public int getStarY(int myY) 
  {//get method
    return starField[1];
  }//end getStarY
 
  public int getStarRadius(int myRadius) 
  {//get method
    return starField[5];
  }//end getStarRadius
 
  public int getStarLayer(int myLayer) 
  {//get method
    return starField[4];
  }//end getStarLayer
 
}//end class star

the runtime errors i get:

 
C:\JSource\StarScape>javac StarScape.java Star/Star.java
 
C:\JSource\StarScape>appletviewer starscape.html
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at StarScape.paint(StarScape.java:32)
        at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
 
        at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:713)
 
        at javax.swing.RepaintManager.seqPaintDirtyRegions(RepaintManager.java:6
93)
        at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(System
EventQueueUtilities.java:125)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:641)
        at java.awt.EventQueue.access$000(EventQueue.java:84)
        at java.awt.EventQueue$1.run(EventQueue.java:602)
        at java.awt.EventQueue$1.run(EventQueue.java:600)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessCo
ntrolContext.java:87)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:611)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
 
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
 
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at StarScape.paint(StarScape.java:32)
        at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
 
        at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:713)
 
        at javax.swing.RepaintManager.seqPaintDirtyRegions(RepaintManager.java:6
93)
        at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(System
EventQueueUtilities.java:125)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:641)
        at java.awt.EventQueue.access$000(EventQueue.java:84)
        at java.awt.EventQueue$1.run(EventQueue.java:602)
        at java.awt.EventQueue$1.run(EventQueue.java:600)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessCo
ntrolContext.java:87)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:611)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
 
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
 
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
 
C:\JSource\StarScape>

I know that there may be better ways to do this, but it is for class. The rules for the project:

1)create a package or class for stars. Stars should have properties like location, size, speed, angle, color.
2)at least 3 colors, basically three starfields/layers on top of one another. Should run as one starfield not three, just look like 3.
2)must have a moveStar setter method; stars must grow and accelerate toward edge of screen as part of the moveStar Method.
3)must have get methods for x and y positions, color/layer, and size
4)main applet must call the methods, and draw the starfield (painting must be done in the main applet. Must use rectangle, other shapes have not been taught yet.
5)keyListener to be learned later, this must run until user exits the window. Even if I know keyListener it is NOT to be used, Teach does not want anyone to be far ahead of anyone else.
6)can use vector math if known (I do, can only use math.sqrt since math.hypot has not been gone over in class) else can move star on the four diagonals (i.e. x-1,y-1 AND x-1,y+1 AND x+1,y+1 AND x+1,y-1)

Please help. Code as been commented for easy read.

**Norm** · August 16th, 2011, 04:12 PM

appletviewer starscape.html
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at StarScape.paint(StarScape.java:32)

There is a null variable at line 32 in your code. Find that variable and back track to see why it has a null value and then change the code so it has a valid value.

   Star stars[] = new Star[maxStars];//make 100 star objects

Here you define an array to hold some Star objects. Now you need to fill the array.
This definition should be outside of the paint method so other code can fill it with the needed Star objects.

**Spidey1980** · August 16th, 2011, 07:29 PM

so I just tried this:

    Star stars[] = new Star[maxStars];//make 100 star objects
    for (index = 0; index < maxStars; index++)
    {
      stars[index] = new Star[];//fill array with stars
    }//end for

and it tells me that

 
C:\JSource\StarScape>javac StarScape.java Star/Star.java
StarScape.java:28: Star() is not public in Star.Star; cannot be accessed from ou
tside package
      stars[index] = new Star();//fill array with stars
                     ^
1 error
 
C:\JSource\StarScape>

lol so i'm getting closer. Thank you so much for your help.

edit: yup I forgot the "public void" infront of star() in my package. This issue is solved, but now I have another issue with this code that does not fit in this thread.

SOLVED so far

**Norm** · August 16th, 2011, 07:32 PM

Yes you could use a loop to fill the array.
In the loop assign each element a new Star object.

**Junky** · August 16th, 2011, 07:59 PM

Star(){

If you do not provide an access modifier it will use the default of package private. This means that only classes in the same package can access it.

**Spidey1980** · August 16th, 2011, 08:36 PM

null pointer and dirty regions issue SOLVED Thank you all for your help.

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