private void checkSubboardIssues(int[][] cells) {
		final int[] set = new int[Board.SIZE];
		for (int i = 0; i < Board.SIZE; i += Board.SUB_BOARD_SIZE) {
			for (int j = 0; j < Board.SIZE; j += Board.SUB_BOARD_SIZE) {

				// check each subboard
				Arrays.fill(set, 0);
				for (int y = i; y < i + Board.SUB_BOARD_SIZE; y++) {
					for (int x = j; x < j + Board.SUB_BOARD_SIZE; x++) {
						if (cells[y][x] > 0) {
							int value = cells[y][x];
							if (set[value - 1] >= 1) {
								throw new IllegalArgumentException("Board is not valid");
							}
							set[value - 1]++;
						}
					}
				}

			}
		}
	}
Checks if board has issues in each sub-board of the board.

@param cells the board
@throws IllegalArgumentException if board contains initial issues
jenetics.example/src/main/java/io/jenetics/example/sudoku/Board.java