/** this class is fully thread-safe */
class Flag {
  private boolean up;

  /** returns true if flag was down before */
  public synchronized boolean raise() {
    return doRaise();
  }

  private synchronized boolean doRaise() {
    if (!up) {
      up = true;
      notifyAll();
      return true;
    } else
      return false;
  }

  public synchronized void waitUntilUp() {
    if (!up) {
      try {
        wait();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

  public synchronized boolean isUp() {
    return up;
  }

  public String toString() {
    return isUp() ? "up" : "down";
  }

  // currently does a semi-active wait with latency = 50 ms
  public void waitForThisOr(Flag otherFlag) ctex {
    while (!isUp() && !otherFlag.isUp())
      Thread.sleep(50);
  }
}

Began life as a copy of #1000781