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Java Academy · Lesson

Collectors & Grouping

Learn how to collect Stream results and group data.

Collectors & Grouping is a free Java Academy lesson on CoddyKit — lesson 3 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, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro

Collectors gather Stream results into containers. Grouping organizes items by category.

toList

Collectors.toList() gathers Stream elements into a List.

Code: toList

This collects uppercase names into a List.

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<String> names = Arrays.asList("Ada","Bob","Eve");
    List<String> upper = names.stream().map(String::toUpperCase).collect(Collectors.toList());
    System.out.println(upper);
  }
}

joining

Collectors.joining() concatenates strings with optional delimiter.

Code: joining

This joins words with spaces to form a sentence.

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<String> words = Arrays.asList("Java","is","fun");
    String sentence = words.stream().collect(Collectors.joining(" "));
    System.out.println(sentence);
  }
}

Code: groupingBy

Collectors.groupingBy groups elements into a Map keyed by a classifier (first letter here).

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<String> names = Arrays.asList("Alice","Bob","Anna");
    Map<Character, List<String>> grouped = names.stream().collect(Collectors.groupingBy(s -> s.charAt(0)));
    System.out.println(grouped);
  }
}

Collectors check

Quick check: Which collector groups elements by a classifier function?

Recap

Recap: Collectors let you gather Stream results. toList collects, joining concatenates, groupingBy categorizes.

Frequently asked questions

Is the “Collectors & Grouping” lesson free?

Yes — the full text of “Collectors & Grouping” is free to read here on the web, and the Java Academy course includes 3 lessons in total. 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.

What will I learn in “Collectors & Grouping”?

Learn how to collect Stream results and group data. 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; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Collectors & Grouping” 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

  1. Stream Sources
  2. map/filter/reduce
  3. Collectors & Grouping
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