Trying to build Sudoku board in Java Swing. NullPointerException error? -

i working on building gui java program generate , solve sudoku puzzles. way doing have 1 main jpanel, 3x3 jpanels inside it, have 3x3 jlabels in them.

here have far:

package sudoku;  import java.awt.gridlayout; import javax.swing.*; import javax.swing.border.border;  public class gui {      gui gui;     jpanel board;     jpanel subboard[][];     gridlayout layout;     jlabel square[][];     border big;     border small;      public void load() {          gui = new gui();         gui.setup();         gui.buildboard();      }      private void setup() {          layout = new gridlayout(3, 3);         board = new jpanel(layout);         subboard = new jpanel[3][3];         square = new jlabel[9][9];      }      private void buildboard() {          // set board         board.setvisible(true);          int mod = 0;         (int = 0; < 3; i++) {             (int j = 0; j < 3; j++) {                 // add subboards board                 board.add(subboard[i][j]);                 subboard[i][j].setlayout(layout);                  // add textfields each subboard                 subboard[i][j].add(square[i + mod][j + mod]);              }             mod += 3;          }     }  } 

i getting nullpointerexception error on line:

board.add(subboard[i][j]); 

can explain why happening? if see other problems, or better way doing please let me know! thanks.

in setup() method @ line

subboard = new jpanel[3][3]; 

you've created array have not filled jpanel's yet.

so, default array contains null every index position , hence when try add them board (which unlike subboard initialized new jpanel(layout)) throws npe.