Practical uses for AtomicInteger

Question

I sort of understand that AtomicInteger and other Atomic variables allow concurrent accesses. In what cases is this class typically used though?

Answer 1

There are two main uses of AtomicInteger:

• As an atomic counter (incrementAndGet(), etc) that can be used by many threads concurrently

• As a primitive that supports compare-and-swap instruction (compareAndSet()) to implement non-blocking algorithms.

  Here is an example of non-blocking random number generator from Brian Göetz's Java Concurrency In Practice:

  public class AtomicPseudoRandom extends PseudoRandom {
      private AtomicInteger seed;
      AtomicPseudoRandom(int seed) {
          this.seed = new AtomicInteger(seed);
      }
  
      public int nextInt(int n) {
          while (true) {
              int s = seed.get();
              int nextSeed = calculateNext(s);
              if (seed.compareAndSet(s, nextSeed)) {
                  int remainder = s % n;
                  return remainder > 0 ? remainder : remainder + n;
              }
          }
      }
      ...
  }
  

  As you can see, it basically works almost the same way as incrementAndGet(), but performs arbitrary calculation (calculateNext()) instead of increment (and processes the result before return).