How to average BigDecimals using Streams?

You don't need to stream twice. Simply call List.size() for the count:

public BigDecimal average(List<BigDecimal> bigDecimals, RoundingMode roundingMode) {
    BigDecimal sum = bigDecimals.stream()
        .map(Objects::requireNonNull)
        .reduce(BigDecimal.ZERO, BigDecimal::add);
    return sum.divide(new BigDecimal(bigDecimals.size()), roundingMode);
}

Alternatively you can use this Collector implementation:

class BigDecimalAverageCollector implements Collector<BigDecimal, BigDecimalAccumulator, BigDecimal> {

    @Override
    public Supplier<BigDecimalAccumulator> supplier() {
        return BigDecimalAccumulator::new;
    }

    @Override
    public BiConsumer<BigDecimalAccumulator, BigDecimal> accumulator() {
        return BigDecimalAccumulator::add;
    }

    @Override
    public BinaryOperator<BigDecimalAccumulator> combiner() {
        return BigDecimalAccumulator::combine;
    }

    @Override
    public Function<BigDecimalAccumulator, BigDecimal> finisher() {
        return BigDecimalAccumulator::getAverage;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return Collections.emptySet();
    }

    @NoArgsConstructor
    @AllArgsConstructor
    static class BigDecimalAccumulator {
        @Getter private BigDecimal sum = BigDecimal.ZERO;
        @Getter private BigDecimal count = BigDecimal.ZERO;

        BigDecimal getAverage() {
           return BigDecimal.ZERO.compareTo(count) == 0 ?
                  BigDecimal.ZERO :
                  sum.divide(count, 2, BigDecimal.ROUND_HALF_UP);
        }

        BigDecimalAccumulator combine(BigDecimalAccumulator another) {
            return new BigDecimalAccumulator(
                    sum.add(another.getSum()),
                    count.add(another.getCount())
            );
        }

        void add(BigDecimal successRate) {
            count = count.add(BigDecimal.ONE);
            sum = sum.add(successRate);
        }
    }

}

And use it like that:

BigDecimal mean = bigDecimals.stream().collect(new BigDecimalAverageCollector());

Note: example uses Project Lombok annotations to shorten the glue code.

BigDecimal[] totalWithCount
                = bigDecimals.stream()
                .filter(bd -> bd != null)
                .map(bd -> new BigDecimal[]{bd, BigDecimal.ONE})
                .reduce((a, b) -> new BigDecimal[]{a[0].add(b[0]), a[1].add(BigDecimal.ONE)})
                .get();
BigDecimal mean = totalWithCount[0].divide(totalWithCount[1], roundingMode);

Optional text description of the code for those that are find that to be helpful (Ignore if you find the code sufficiently self explanatory.):

• The list of BigDecimals is converted to a stream.
• null values are filtered out of the stream.
• The stream of BigDecimals is mapped to as stream of two element arrays of BigDecimal where the first element is the element from the original stream and the second is the place holder with value one.
• In the reduce the a of (a,b) value has the partial sum in the first element and the partial count in the second element. The first element of the b element contains each of the BigDecimal values to add to the sum. The second element of b is not used.
• Reduce returns an optional that will be empty if the list was empty or contained only null values.
  • If the Optional is not empty, Optional.get() function will return a two element array of BigDecimal where the sum of the BigDecimals is in the first element and the count of the BigDecimals is in the second.
  • If the Optional is empty, NoSuchElementException will be thrown.
• The mean is computed by dividing the sum by the count.

I use the above method in order to get the average of a list of BigDecimal objects. The list allows null values.

public BigDecimal bigDecimalAverage(List<BigDecimal> bigDecimalList, RoundingMode roundingMode) {
    // Filter the list removing null values
    List<BigDecimal> bigDecimals = bigDecimalList.stream().filter(Objects::nonNull).collect(Collectors.toList());

    // Special cases
    if (bigDecimals.isEmpty())
        return null;
    if (bigDecimals.size() == 1)
        return bigDecimals.get(0);

    // Return the average of the BigDecimals in the list
    return bigDecimals.stream().reduce(BigDecimal.ZERO, BigDecimal::add).divide(new BigDecimal(bigDecimals.size()), roundingMode);
}