Terminal Operations and reduce

Terminal Operations

Operation	Returns
`toList()` (Java 16), `collect(...)`	a collection / map / string
`forEach`, `forEachOrdered`	nothing (side effect)
`count()`, `min`, `max`	long / Optional
`anyMatch`, `allMatch`, `noneMatch`	boolean (short-circuit)
`findFirst`, `findAny`	Optional
`reduce`	combined single value
`toArray`	array

reduce

reduce(identity, accumulator) combines all elements into one value. The identity is the starting value (0 for sum, 1 for product, "" for concatenation). Without an identity, reduce returns an Optional because the stream could be empty.

Example

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<Integer> nums = List.of(4, 8, 15, 16, 23, 42);

        int sum = nums.stream().reduce(0, Integer::sum);
        int product = nums.stream().limit(3).reduce(1, (a, b) -> a * b);
        Optional<Integer> max = nums.stream().reduce(Integer::max);
        Optional<Integer> none = List.<Integer>of().stream().reduce(Integer::sum);

        System.out.println("sum = " + sum);
        System.out.println("product of first 3 = " + product);
        System.out.println("max = " + max.orElseThrow());
        System.out.println("empty reduce = " + none);
        System.out.println("count > 10: " + nums.stream().filter(n -> n > 10).count());
        System.out.println("any even? " + nums.stream().anyMatch(n -> n % 2 == 0));
        System.out.println("all < 50? " + nums.stream().allMatch(n -> n < 50));
        System.out.println("min = " + nums.stream().min(Integer::compare).get());
    }
}

Output

sum = 108
product of first 3 = 480
max = 42
empty reduce = Optional.empty
count > 10: 4
any even? true
all < 50? true
min = 4

Stream Reuse Error

Example

A stream can be consumed only once.

import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        Stream<String> s = Stream.of("a", "b");
        System.out.println(s.count());
        System.out.println(s.count());
    }
}

Output

2
Exception: java.lang.IllegalStateException: stream has already been operated upon or closed