java - Painting array of constant size? -

i having trouble doing simple, painting array of elements. here have paint method in class array: (xb , yb x , y values want increment make instances show in different locations).

public void paint(graphics pane) {     private box[] boxes = new box[num_box];      for(int = 0; i<num_box; i++){         if (xb == 290){             xb = 0;             yb = yb + 20;         }          boxes[i].paint(pane, xb, yb);         xb = xb + 20;     } 

and here have in box class being painted:

public class box {      private final int width = 20;     private final int height = 20;     private boolean = true;      public void paint(graphics pane, int x, int y) {             pane.setcolor(color.black);                  pane.drawrect(x, y, width, height);          pane.setcolor(color.gray);                    pane.fill3drect(x +2, y+2, width - 3, height - 3, up);       } } 

every time run it, tells me there nullpointerexception @ boxes[i].paint(pane,xb,yb) line. doing wrong?