calculate business days including holidays

Since the accepted answer still uses the obsolete Calendar class – here’s my two cents using the newer Java Date and Time API available in the java.time package.

You have to get the dates of the holidays from somewhere, there is no standard Java library for it. That would be too localized anyway, since holidays heavily depend on your country and region (except for widely known holidays, such as Christmas or Easter).
You could get them from a holiday API, for instance. In the code below, I’ve hardcoded them as a Set of LocalDates.

Java 9

LocalDate startDate = LocalDate.of(2012, 3, 7);
LocalDate endDate = LocalDate.of(2012, 6, 7);

// I've hardcoded the holidays as LocalDates
// and put them in a Set
final Set<LocalDate> holidays = Set.of(
    LocalDate.of(2018, 7, 4)
);
// For the sake of efficiency, I also put the business days into a Set.
// In general, a Set has a better lookup speed than a List.
final Set<DayOfWeek> businessDays = Set.of(
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY
);

List<LocalDate> allDates =

    // Java 9 provides a method to return a stream with dates from the
    // startdate to the given end date. Note that the end date itself is
    // NOT included.
    startDate.datesUntil(endDate)

        // Retain all business days. Use static imports from
        // java.time.DayOfWeek.*
        .filter(t -> businessDays.contains(t.getDayOfWeek()))

        // Retain only dates not present in our holidays list
        .filter(t -> !holidays.contains(t))

         // Collect them into a List. If you only need to know the number of
         // dates, you can also use .count()
        .collect(Collectors.toList());

Java 8

The method LocalDate.datesUntil is not available in Java 8, so you have to obtain a stream of all dates between those two dates in a different manner. We must first count the total number of days in between using the ChronoUnit.DAYS.between method.

long numOfDaysBetween = ChronoUnit.DAYS.between(startDate, endDate);

Then we must generate a sequence of integers exactly as long as the number of days between the start and end date, and then create LocalDates from it, starting at the start date.

IntStream.iterate(0, i -> i + 1)
    .limit(numOfDaysBetween)
    .mapToObj(startDate::plusDays)

Now we have a Stream<LocalDate>, and you can then use the remaining part of the Java 9 code. You also need to replace the usage of Set.of(), because it’s not available in Java 8. Probably by new HashSet<>(Arrays.asList(MONDAY...FRIDAY)).