Java Academy · Lesson

List & Set Overview

Understand List vs Set: ordering, duplicates, and everyday operations. We will add, remove, search, and iterate with simple examples.

Lesson 1 of 38 steps

List & Set Overview is a free Java Academy lesson on CoddyKit. This is lesson 1 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.

What are collections?

Collections hold groups of objects. List is ordered and index-based; Set focuses on membership and uniqueness. Pick based on the question you ask: order or uniqueness?

List vs Set — keys

List: keeps insertion order, duplicates allowed, access by index.
Set: no duplicates; order not guaranteed in HashSet.
Choose List for sequences; choose Set to avoid duplicates and for fast membership checks.

Code: ArrayList basics

ArrayList keeps order and allows duplicates. You can access by index and remove by value or index.

public class Main {
  public static void main(String[] args) {
    java.util.List<String> items = new java.util.ArrayList<>();
    // add elements (duplicates allowed)
    items.add("apple");
    items.add("banana");
    items.add("apple"); // duplicate ok

    // access by index
    System.out.println(items.get(0)); // apple

    // remove by value and by index
    items.remove("banana");
    items.remove(0); // remove first

    // print remaining
    System.out.println(items); // [apple]
  }
}

Everyday operations

Common operations:

add(e), remove(e), contains(e)
size() to get the count
Iterate with index or enhanced for

Code: HashSet uniqueness

HashSet removes duplicates and makes membership checks easy. Printing may show elements in any order.

public class Main {
  public static void main(String[] args) {
    java.util.Set<String> tags = new java.util.HashSet<>();
    tags.add("java");
    tags.add("oop");
    tags.add("java"); // duplicate ignored

    System.out.println("size=" + tags.size()); // 2

    // membership check
    System.out.println("has oop? " + tags.contains("oop"));
    System.out.println(tags); // order not guaranteed
  }
}

Code: iteration patterns

Iterate by index when you need positions; use enhanced for for readability when you only need elements.

public class Main {
  public static void main(String[] args) {
    java.util.List<String> list = new java.util.ArrayList<>();
    list.add("A"); list.add("B"); list.add("C");

    // classic for with index
    for (int i = 0; i < list.size(); i = i + 1) {
      System.out.print(list.get(i) + (i < list.size()-1 ? " " : "\n"));
    }

    // enhanced for (for-each)
    for (String s : list) {
      System.out.print("[" + s + "]");
    }
    System.out.println();
  }
}

List vs Set check

Quick check: Which statement is true about List and Set in Java?

Answer it in the interactive lesson

Recap

Recap: Use List for ordered sequences and Set for unique items and membership checks. Practice: convert a List to a Set to remove duplicates, then back if you need order.

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Frequently Asked Questions

Is the “List & Set Overview” lesson free?

Yes — the full text of “List & Set Overview” 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 “List & Set Overview”?

Understand List vs Set: ordering, duplicates, and everyday operations. We will add, remove, search, and iterate with simple examples. 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 1 of 3.

How long does the “List & Set Overview” 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.

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