Java Technical Tutorial

Java Stream API: Functional Pipelines, Transformations & Reductions

Learn Java 8+ Stream API with practical examples: filter, map, flatMap, sorted, distinct, reduce, Collectors.groupingBy, and parallel streams.

Java

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class StreamApiTutorial {
    public record Developer(String name, String skill, int yearsExp) {}

    public static void main(String[] args) {
        List<Developer> devs = Arrays.asList(
            new Developer("Vikas", "Java", 6),
            new Developer("Sneha", "Python", 4),
            new Developer("Karan", "Java", 8),
            new Developer("Pooja", "React", 3)
        );

        // Group senior developers by primary skill
        Map<String, List<Developer>> groupedBySkill = devs.stream()
            .filter(d -> d.yearsExp() >= 4)
            .collect(Collectors.groupingBy(Developer::skill));

        groupedBySkill.forEach((skill, list) -> {
            System.out.println(skill + ": " + list.stream().map(Developer::name).toList());
        });
    }
}

1. Streams vs. Collections

A collection is an in-memory data structure that holds elements, while a Stream is a computational pipeline that processes elements on-demand. Streams do not mutate their underlying source; they yield transformations lazily.

2. Intermediate vs. Terminal Operations

Stream pipelines consist of two distinct types of operations:

Intermediate Operations: (e.g. filter(), map(), flatMap(), sorted()) Return another Stream and execute lazily. No computation happens until a terminal operation is called.
Terminal Operations: (e.g. collect(), forEach(), reduce(), count()) Trigger traversal of the pipeline and produce a concrete result or side-effect.

Key Engineering Takeaways

Streams are single-use; once a terminal operation executes, the stream cannot be reused.
Prefer method references (Class::method) for concise functional code.
Use parallelStream() judiciously only for CPU-bound computations on large datasets.

Frequently Asked Questions

What is the difference between map() and flatMap()?

map() transforms each element into a single value, while flatMap() transforms each element into a stream and flattens multiple streams into a single consolidated stream.

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