@Override
    public String toString(){
        StringBuffer result = new StringBuffer();
        for (int i = 0; i < rowNum; i++) {
            result.append("[ ");
            for (int j = 0; j < columnNum; j++) {
                hexString(result,i,j);
            }
            result.append(" ]");
            result.append("\n");


        }
        return result.toString();
    }