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Java Academy · Lesson

Atomic & Concurrent Collections

Learn to use atomic variables and concurrent collections for thread safety.

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

Intro

Atomic variables and concurrent collections simplify thread-safe programming.

AtomicInteger

AtomicInteger offers atomic increment, decrement, and compare-and-set operations.

Code: AtomicInteger

AtomicInteger ensures updates are thread-safe without explicit locks.

import java.util.concurrent.atomic.*;public class Main {
  public static void main(String[] args) throws Exception {
    AtomicInteger count = new AtomicInteger(0);
    Thread t1 = new Thread(() -> { for(int i=0;i<1000;i++) count.incrementAndGet(); });
    Thread t2 = new Thread(() -> { for(int i=0;i<1000;i++) count.incrementAndGet(); });
    t1.start(); t2.start();
    t1.join(); t2.join();
    System.out.println("Count = " + count.get());
  }
}

Concurrent collections

Concurrent collections like ConcurrentHashMap or CopyOnWriteArrayList handle concurrent access safely.

Code: ConcurrentHashMap

ConcurrentHashMap allows safe concurrent writes and reads.

import java.util.concurrent.*;public class Main {
  public static void main(String[] args) throws Exception {
    ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
    Thread t1 = new Thread(() -> { map.put("A", 1); });
    Thread t2 = new Thread(() -> { map.put("B", 2); });
    t1.start(); t2.start();
    t1.join(); t2.join();
    System.out.println(map);
  }
}

Code: CopyOnWriteArrayList

CopyOnWriteArrayList avoids ConcurrentModificationException during iteration.

import java.util.concurrent.*;public class Main {
  public static void main(String[] args) {
    CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
    list.add("Hello");
    list.add("World");
    for(String s : list) {
      list.add("Extra"); // safe even during iteration
      System.out.println(s);
    }
  }
}

Atomic/Concurrent check

Quick check: Why use an AtomicInteger instead of a normal int with multiple threads?

Recap

Recap: Use atomic classes for thread-safe counters and concurrent collections for safe shared data structures.

Frequently asked questions

Is the “Atomic & Concurrent Collections” lesson free?

Yes — the full text of “Atomic & Concurrent Collections” 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 “Atomic & Concurrent Collections”?

Learn to use atomic variables and concurrent collections for thread safety. 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 2 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Atomic & Concurrent Collections” 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. Synchronization & Locks
  2. Atomic & Concurrent Collections
  3. Common Concurrency Bugs
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