Can you give an example of reducer functions used in Stream API for data processing?

In the Stream API, reduce functions (reducers) are used to fold stream elements into a single value. Main examples:

• reduce(BinaryOperator<T> accumulator) — folds elements by applying a binary operation.
• reduce(T identity, BinaryOperator<T> accumulator) — folding with an initial value.

Example: summing numbers in a list

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.stream()
                 .reduce(0, (a, b) -> a + b);
System.out.println(sum); // 15

Another example — concatenating strings:

List<String> words = Arrays.asList("Java", "Stream", "API");
String result = words.stream()
                     .reduce("", (a, b) -> a + b + " ").trim();
System.out.println(result); // "Java Stream API"

Thus, reducers allow aggregating data from a stream into a single final value.