How to Sum BigDecimal Values with Java Stream API and Reduce

1. Introduction

We usually use Java Stream API to process collections. A useful feature is the sum operation for numeric streams, but it cannot handle all numeric types.

This article shows how to perform sum operations on streams of types such as BigDecimal.

2. How to Sum with Streams

Stream API provides numeric streams: IntStream, DoubleStream, and LongStream. Example using IntStream#sum:

IntStream intNumbers = IntStream.range(0, 3);
assertEquals(3, intNumbers.sum());

Similarly, we can sum a list of Doubles by converting the object stream to a DoubleStream with mapToDouble :

List<Double> doubleNumbers = Arrays.asList(23.48, 52.26, 13.5);
double result = doubleNumbers.stream()
    .mapToDouble(Double::doubleValue)
    .sum();
assertEquals(89.24, result, .1);

We would like the same approach for a collection of BigDecimal values.

3. Using Reduce for BigDecimal

We use Stream.reduce to compute the sum:

Stream<Integer> intNumbers = Stream.of(5, 1, 100);
int result = intNumbers.reduce(0, Integer::sum);
assertEquals(106, result);

This works for any data that can be logically added, including BigDecimal:

Stream<BigDecimal> bigDecimalNumber = Stream.of(BigDecimal.ZERO, BigDecimal.ONE, BigDecimal.TEN);
BigDecimal result = bigDecimalNumber.reduce(BigDecimal.ZERO, BigDecimal::add);
assertEquals(11, result);

The reduce method takes two parameters:

Identity – the initial value, e.g., 0, which serves as the starting point of the reduction.

Accumulator function – a function that accepts the current accumulated result and the next element from the stream.

4. Conclusion

In this article we examined how to calculate the sum of numbers in numeric streams and how to use reduce as an alternative, especially for types like BigDecimal that lack a dedicated stream implementation.