`volatile` vs `synchronized` — what problem does each solve?

Answer

`volatile` guarantees visibility (and ordering) of a variable between threads, but doesn’t make compound operations atomic. `synchronized` provides mutual exclusion and also establishes happens-before (visibility) for the protected code.

class StopFlag {
  private volatile boolean stop = false;

  void requestStop() {
    stop = true;
  }

  void runLoop() {
    while (!stop) {
      // do work
    }
  }
}