Java Academy · Lesson

map/filter/reduce

Learn core Stream operations: map, filter, reduce.

Lesson 2 of 38 steps

map/filter/reduce is a free Java Academy lesson on CoddyKit. This is lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, and your progress syncs across the web and the CoddyKit app. The Java Academy course includes 3 lessons in total.

Intro

map transforms data, filter selects items, reduce combines results into one.

map

map: apply a function to each element to produce a new Stream.

Code: map

This maps each string to uppercase.

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<String> names = Arrays.asList("ada", "bob");
    names.stream().map(String::toUpperCase).forEach(System.out::println);
  }
}

filter

filter: keep only elements that match a condition.

Code: filter

This filters even numbers from a list.

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<Integer> nums = Arrays.asList(1,2,3,4,5);
    nums.stream().filter(n -> n % 2 == 0).forEach(System.out::println);
  }
}

Code: reduce

reduce combines elements, here summing numbers.

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<Integer> nums = Arrays.asList(1,2,3,4);
    int sum = nums.stream().reduce(0, (a,b) -> a+b);
    System.out.println("Sum = " + sum);
  }
}

map/filter/reduce check

Quick check: Which Stream operation combines elements into a single result?

Answer it in the interactive lesson

Recap

Recap: map transforms, filter selects, reduce combines. These are core Stream tools.

Free to start

Learn Java with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses: 104
Lessons: 374

Start learning free View the full course

Frequently Asked Questions

Is the “map/filter/reduce” lesson free?

Yes — the full text of “map/filter/reduce” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Java Academy course, upgrade to CoddyKit PRO. The Java Academy course includes 3 lessons in total.

What will I learn in “map/filter/reduce”?

Learn core Stream operations: map, filter, reduce. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Java Academy?

No prior experience is required. Java Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 3.

How long does the “map/filter/reduce” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Java Academy lesson?

Yes. Every Java Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Java Academy