Comprehensive Guide to Java 8 Stream API with Practical Examples

Java 8 introduced the Stream API and Lambda expressions, allowing developers to write concise, functional‑style operations on collections.

Streams treat a source of elements as a pipeline, supporting intermediate operations (filter, map, flatMap, sorted, distinct) that return a new stream, and terminal operations (forEach, collect, reduce, count, max, min) that produce a result or side‑effect.

Key characteristics of streams include non‑storage of data, immutability of the source, and lazy execution—intermediate operations are performed only when a terminal operation is invoked.

Streams can be created from collections, arrays, or static methods such as Stream.of(...) , Arrays.stream(array) , and list.stream() (or list.parallelStream() for parallel execution).

Common examples:

List
list = Arrays.asList(6,7,3,8,1,2,9);
list.stream().filter(x -> x > 7).forEach(System.out::println); // prints 8 9

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

Optional
longest = strings.stream()
    .max(Comparator.comparingInt(String::length));
System.out.println("Longest: " + longest.get());

Reduction operations such as reduce can compute sums, products, or custom aggregates, while collect offers a rich set of collectors for converting streams into lists, sets, maps, or statistical summaries.

Grouping and partitioning enable categorising data, e.g., personList.stream().collect(Collectors.groupingBy(Person::getSex)) or partitioningBy(p -> p.getSalary() > 8000) .

Finally, streams support concatenation, distinct, limit, and skip to combine or trim data streams.