reduce
- Java course
- Advanced
- Lesson 4/82
- 5 min
Learn accumulation with reduce.
Reducing a stream to one value: reduce
reduce combines all elements of a stream into a single result. The two-argument form takes an identity value and a BinaryOperator<T>; the result is always of type T.
Example: prices.stream().reduce(0.0, Double::sum). Here 0.0 is the starting value and Double::sum is the operator that adds two values. For an empty stream the result is the identity; this makes it safe to use, unlike the form that returns an Optional.
The three-argument reduce
The three-argument reduce form takes an identity, an accumulator and a combiner. You use it when the result type differs from the stream element, for example reducing a Stream<String> to an int count.
Example: words.stream().reduce(0, (sum, w) -> sum + w.length(), Integer::sum). Here 0 is the identity, the second argument is the accumulator that adds each string's length, and the third is the combiner that merges partial results in a parallel stream. This form is also safe for serial streams.
Practice Sum prices with reduce
Create a List<Double> of values 12.5, 25.0 and 5.0. Sum the stream with reduce(0.0, Double::sum) and print the result with System.out.println.
Starter code
import java.util.List;
// Sum prices with reduce
// Start from the 0.0 identity
// Print the totalSample solution
import java.util.List;
public class SumPrices {
public static void main(String[] args) {
List<Double> prices = List.of(12.5, 25.0, 5.0);
double sum = prices.stream().reduce(0.0, Double::sum);
System.out.println(sum);
}
}Expected output
42.5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the reduce call
double sum = prices.stream()
.reduce(____, Double::sum);double sum = prices.stream()
.reduce(0.0, Double::sum);Explanation: The identity for addition is 0.0.
2 Match the reduce overloads
| Term | Meaning |
|---|---|
reduce(BinaryOperator) |
Returns Optional<T> |
reduce(identity, op) |
Returns T |
reduce(id, acc, comb) |
Returns a different type |
3 Which reduce overload returns Optional<T>?
- reduce(identity, BinaryOperator)
- reduce(BinaryOperator) Correct answer
- reduce(identity, acc, comb)
- reduce(identity, mapper)
Explanation: The single-argument reduce(BinaryOperator) returns Optional<T>.