Timer (Java)

/*
 * The class Timer and its constructor may be
 * used to produces instances of objects of 
 * type Timer. An object of type Timer has
 * methods start() and stop() that start
 * and stop a timer, a method called reset
 * that resets a timer, and a method called
 * elaspedTimeInMs that returns an amount of
 * elapsed time. If a timer has been started
 * but not stopped, then elapsedTimeInMs
 * returns the amount of time (in milliseconds)
 * that has elapsed since the timer was started.
 * If a timer has been started and stopped,
 * then elapsedTimeInMs returns the amount of
 * time that elapsed between the time the
 * timer was started and stopped.
 */
class Timer {
  long timeStarted;
  long timeStopped;
  Timer() {
    this.reset();
  }
  void reset() {
    this.timeStarted = 0;
    this.timeStopped = 0;
  }
  void start() {
    this.timeStarted = System.currentTimeMillis();
  }
  void stop() {
    this.timeStopped = System.currentTimeMillis();
  }
  long elapsedTimeInMs() {
    if (this.timeStarted != 0) {
      if (this.timeStopped != 0) {
        return this.timeStopped - this.timeStarted;
      }
      else {
        return System.currentTimeMillis() - this.timeStarted;
      }
    }
    else {
      return -1;
    }
  }
}