stream.forEach() / reduce() / count()

// Performs an action on each element (terminal operation).
stream.forEach(element -> action);

// Aggregates elements (identity, (accumulator, element) -> new accumulator).
stream.reduce(identity, (a, b) -> operation);

// Returns the number of elements.
stream.count();

// Returns the sum of a numeric stream.
intStream.sum();

// Returns the average of a numeric stream (OptionalDouble).
intStream.average();

import java.util.Arrays;
import java.util.List;

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

// Use forEach to print all elements.
numbers.stream()
    .forEach(n -> System.out.println(n)); // Prints 1 through 5.

// You can also use a method reference for brevity.
numbers.stream().forEach(System.out::println);

// Use reduce to compute the sum (identity 0, (sum, element) -> sum + element).
int sum = numbers.stream()
    .reduce(0, (a, b) -> a + b);
System.out.println(sum); // Prints 15.

// Use reduce to find the maximum value.
int max = numbers.stream()
    .reduce(Integer.MIN_VALUE, (a, b) -> a > b ? a : b);
System.out.println(max); // Prints 5.

// Use count to check the number of matching elements.
long count = numbers.stream()
    .filter(n -> n % 2 == 0)
    .count();
System.out.println(count); // Prints 2 (the count of even numbers).

// Convert to a numeric stream with mapToInt, then use sum and average.
int total = numbers.stream().mapToInt(Integer::intValue).sum();
System.out.println(total); // Prints 15.

double avg = numbers.stream().mapToInt(Integer::intValue).average().orElse(0);
System.out.println(avg); // Prints 3.0.

forEach() behaves similarly to an enhanced for loop on a collection, but its key advantage is that it can be chained with intermediate stream operations. You cannot modify local variables inside forEach() — they must be effectively final.

reduce() is a general-purpose aggregation operation suitable for computing sums, products, maximum values, string concatenation, and more. That said, for common operations like sums and maximums, using dedicated methods such as sum() and max() on IntStream improves readability.

To collect a stream into a collection, see collect() / toList().