Paint component doesn't print polygon according to intended coordinate

Question

I have problem drawing the triangle polygons based on the coordinates which are stored in linked list. When I checked the linked list elements using System.out.println in paint component method

Public void paintComponent (Graphics g)  {
...
   for (Polygon p : triangles) {
      g2.drawPolygon(triangle); 
      ... // print the elements of triangles
      System.out.println(p.xpoints[0]);
      System.out.println(p.xpoints[1]);
      System.out.println(p.xpoints[2]);
   }
}

it is not similar with the linked list elements read in

public void getTriangles (){
   .....
   while (overlap) 
   {
    ...
   }

   for (Polygon p: triangles){
      ... //print the elements of triangles
      System.out.println(p.xpoints[0]);
      System.out.println(p.xpoints[1]);
      System.out.println(p.xpoints[2]);
   } 
}

I was wondering WHY this happened. e.g. the x points in linked list triangles read in public getTriangles method are x[0]= 379, x[0]= 429, x[2]= 404, and in paintComponent(Graphics g)x[0]= 249, x[0]= 299, x[2]= 274

   public class patternGenerator extends JPanel {
       private int n = 10;
       private int[] xCoord = new int[100];
       private int[] yCoord = new int[100]; 

       private List<Polygon> triangles = new LinkedList<Polygon>(); 

       public patternGenerator ()  {
       ....
       getTriangles();
       }

       public void getTriangles (){
          boolean overlap = true; //overlap


          /* This LOOP check for overlapping triangle. 
           * It will erase previous elements in linked list which are overlapping.
           * The triangles will only be painted when there is no overlapping triangles
           */
          while(overlap) {
             int MAX_WIDTH  = 600; 
             int MAX_HEIGHT = 600;
             int sizeCombination2a [] = {10,20};

             //remove previous drawn triangles
             if(triangles.size()>1) {
                Polygon[] triArray = triangles.toArray(new Polygon[triangles.size()]);
                for (Polygon p:triArray)  
                   triangles.remove (p);
             }

            //create new triangles
            for(int i = 0; i < numTriangles; i++) { //generate 10 triangles {

               int xWidth = sizeCombination2a[generator.nextInt(sizeCombination2a.length)]
               int yHeight= sizeCombination2a[generator.nextInt(sizeCombination2a.length)];

               xCoord[0] = generator.nextInt(MAX_WIDTH);
               xCoord[1] = (int) (xCoord[0] - xWidth);
               xCoord[2] = (int) (xCoord[1] + (xWidth/2));

               yCoord[0] = generator.nextInt(MAX_WIDTH);    
               yCoord[1] = yCoord[0];
               yCoord[2] = (int) (yCoord[1] - yHeight);  

               triangles.add( new Polygon(xCoord,yCoord, 3)); 
            }
            boolean results = checkOverlapAxis(aesParameter,aesLevel);

            if(results)
               overlap = false;

        }//end while (overlap)
     }

     public void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g; 
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setStroke(new BasicStroke(1)); // set the thickness of polygon line
        g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 1.00f));
        g2.setPaint(Color.black);//set the polygon line 

        for (Polygon triangle : triangles)
           g2.drawPolygon(triangle);

     }//end Paint
 }

Answer 1

This could be a threading problem. Make sure all activity with this triangles variable is synchronized.

Better still, restrict all activity (setting and getting and painting) with the triangles variable to the event dispatch thread, as so:

SwingUtilities.invokeLater(new Runnable() {
    public void run() {
        patternGenerator.setTriangles(...);
   }
}

Answer 2

Finally I solved this problem by declaring the LIST as:

public static List<Polygon> triangles = new LinkedList<Polygon>();