Aggregation of CSV rows using Java-8 functions

CSVAggregator.java

    /**
     * Finds a unique result in the given file, based on examining rows from the CSV and comparing
     * them sequentially down the file using the given value aggregator function.
     * 
     * @param <T>
     *            The type of the value that is being compared.
     * @param <R>
     *            The type of the value that is being returned as the aggregation.
     * @param nextFile
     *            The file to check
     * @param valueAggregator
     *            A {@link BiFunction} which compares values and returns another value based on the
     *            initial value and the value for the current row in the CSV. The initial value must
     *            be explicitly allowed for as the first case.
     * @param initialResultValue
     *            The initial result value to use, and the default to return, if the file is empty.
     * @param valueFromHeadersAndLine
     *            A {@link BiFunction} which takes a List of Strings as the headers, and a line
     *            afterwards and returns the desired value for comparison for that line.
     * @return A value aggregated across the entire file, based on sequential processing of each of
     *         the lines, without holding the entire file in memory at any time.
     * @throws IOException
     *             If there is an exception handling the csv file
     */
    public static <T, R> R aggregateRows(final Path nextFile, final BiFunction<R, T, R> valueAggregator,
            final BiFunction<List<String>, String[], T> valueFromHeadersAndLine, final R initialResultValue)
        throws IOException
    {
        R result = initialResultValue;
        try (final CSVReader reader = new CSVReader(Files.newBufferedReader(nextFile, StandardCharsets.UTF_8));)
        {
            List<String> headers = null;
            String[] nextLine;
            while((nextLine = reader.readNext()) != null)
            {
                if(headers == null)
                {
                    // header line is mandatory at this point as the first line
                    headers = Arrays.asList(nextLine);
                }
                else
                {
                    final T nextValue = valueFromHeadersAndLine.apply(headers, nextLine);
                    // NOTE: The initialResultValue, if mutable, may be modified in the method, and
                    // the same reference returned as the result
                    result = valueAggregator.apply(result, nextValue);
                }
            }
        }
        return result;
    }