Module 13: Functional Programming and Streams

Filter, Map, and Other Intermediate Operations

Filter, map, and reshape data with clear stream pipelines.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-17

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, manually edited for clarity, and checked against current Java documentation and runnable examples.

Learning goals

Use `filter`, `map`, and `sorted` confidently
Read a stream pipeline from left to right
Keep pipelines understandable instead of clever

Intermediate operations build the pipeline: filter removes items, map transforms them, and sorted reorders them.

Read pipelines in stages: source, filter, transform, collect. That mental sequencing helps a lot when debugging.

Keep lambdas small: If the lambda logic gets dense, extract a method reference or named helper.

Do not cram everything into one chain if it hurts readability: Streams are a tool, not a contest.

Runnable examples

Filter then transform

import java.util.List;

List<String> result = List.of("ada", "grace", "linus").stream()
    .filter(name -> name.length() > 3)
    .map(String::toUpperCase)
    .toList();

System.out.println(result);

Expected output

[GRACE, LINUS]

Mini exercise

Take a list of numbers, keep only even values, then multiply each remaining number by 10.

Summary

`filter` removes items, `map` transforms them.
Readable pipelines beat overly dense one-liners.
Stage-by-stage thinking makes streams easier to debug.

Next step

Finish the data flow by learning terminal operations such as `toList`, `count`, and `reduce`.

Sources used

Filter, Map, and Other Intermediate Operations

Filter then transform

What does `map` do in a stream pipeline?