Comprehensive Guide to Java Stream API with Practical Examples

This tutorial introduces the Java 8 Stream API, explaining how streams treat collections as pipelines of data that can be processed lazily.

It shows how to create streams from collections or arrays:

List<String> list = Arrays.asList("a", "b", "c");
Stream<String> stream = list.stream();
Stream<Integer> intStream = Stream.of(1, 2, 3);

Common intermediate operations are demonstrated, including filter to select elements, map to transform them, and flatMap to flatten nested structures:

List<Person> highEarners = personList.stream()
    .filter(p -> p.getSalary() > 8000)
    .collect(Collectors.toList());
List<String> names = personList.stream()
    .map(Person::getName)
    .collect(Collectors.toList());

Terminal operations such as forEach, findFirst, anyMatch, max, min, and count are illustrated with simple examples.

The article also covers aggregation with reduce and the richer collect API, showing how to gather results into lists, sets, maps, and how to perform statistical summaries:

Map<String, Person> highSalaryMap = personList.stream()
    .filter(p -> p.getSalary() > 8000)
    .collect(Collectors.toMap(Person::getName, p -> p));
Long total = personList.stream().collect(Collectors.counting());
Double avg = personList.stream().collect(Collectors.averagingDouble(Person::getSalary));

Grouping and partitioning are explained using Collectors.groupingBy and Collectors.partitioningBy to categorize employees by gender, region, or salary thresholds.

Sorting is demonstrated with both natural ordering and custom comparators, including multi‑level sorting (salary then age):

List<String> sortedBySalary = personList.stream()
    .sorted(Comparator.comparing(Person::getSalary).reversed())
    .map(Person::getName)
    .collect(Collectors.toList());

Additional stream utilities such as distinct, limit, skip, and concat are presented for deduplication, pagination, and merging streams.

Throughout the guide, numerous complete code snippets illustrate each concept, making it a practical reference for developers working with Java collections and functional-style processing.