Java Streams API Complete Guide

What is the Streams API?

Introduced in Java 8, the Streams API lets you process collections in a functional, declarative style — filter, map, reduce — without manual loops.

Basic Pipeline

List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);

int sum = nums.stream()
    .filter(n -> n % 2 == 0)   // keep evens
    .mapToInt(n -> n * 10)     // transform
    .sum();                    // 2*10 + 4*10 + 6*10 = 120

Common Operations

Operation	Does
filter	Keep matching elements
map	Transform each element
collect	Gather into a list/set/map
reduce	Combine into one value
sorted	Sort the stream

Collecting Results

List<String> names = people.stream()
    .filter(p -> p.getAge() > 18)
    .map(Person::getName)
    .collect(Collectors.toList());

FAQs

Are streams faster than loops?

Not necessarily — they prioritise readability. parallelStream() can speed up large datasets.

Can I reuse a stream?

No — a stream is consumed once. More in our Java guides.