Class Stopwatch

  • @GwtCompatible(emulated=true)
    public final class Stopwatch
    extends Object
    An object that accurately measures elapsed time: the measured duration between two successive readings of "now" in the same process.

    In contrast, wall time is a reading of "now" as given by a method like System.currentTimeMillis(), best represented as an Instant. Such values can be subtracted to obtain a Duration (such as by Duration.between), but doing so does not give a reliable measurement of elapsed time, because wall time readings are inherently approximate, routinely affected by periodic clock corrections. Because this class (by default) uses System.nanoTime(), it is unaffected by these changes.

    Use this class instead of direct calls to System.nanoTime() for two reasons:

    • The raw long values returned by nanoTime are meaningless and unsafe to use in any other way than how Stopwatch uses them.
    • An alternative source of nanosecond ticks can be substituted, for example for testing or performance reasons, without affecting most of your code.

    Basic usage:

     Stopwatch stopwatch = Stopwatch.createStarted();
     stopwatch.stop(); // optional
     Duration duration = stopwatch.elapsed();
"time: " + stopwatch); // formatted string like "12.3 ms"

    The state-changing methods are not idempotent; it is an error to start or stop a stopwatch that is already in the desired state.

    When testing code that uses this class, use createUnstarted(Ticker) or createStarted(Ticker) to supply a fake or mock ticker. This allows you to simulate any valid behavior of the stopwatch.

    Note: This class is not thread-safe.

    Warning for Android users: a stopwatch with default behavior may not continue to keep time while the device is asleep. Instead, create one like this:

          new Ticker() {
            public long read() {
              return android.os.SystemClock.elapsedRealtimeNanos();
    Kevin Bourrillion
    • Method Detail

      • createUnstarted

        public static Stopwatch createUnstarted​(Ticker ticker)
        Creates (but does not start) a new stopwatch, using the specified time source.
      • createStarted

        public static Stopwatch createStarted​(Ticker ticker)
        Creates (and starts) a new stopwatch, using the specified time source.
      • isRunning

        public boolean isRunning()
        Returns true if start() has been called on this stopwatch, and stop() has not been called since the last call to start().
      • elapsed

        public long elapsed​(TimeUnit desiredUnit)
        Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit, with any fraction rounded down.

        Note: the overhead of measurement can be more than a microsecond, so it is generally not useful to specify TimeUnit.NANOSECONDS precision here.

        It is generally not a good idea to use an ambiguous, unitless long to represent elapsed time. Therefore, we recommend using elapsed() instead, which returns a strongly-typed Duration instance.

        14.0 (since 10.0 as elapsedTime())
      • toString

        public String toString()
        Returns a string representation of the current elapsed time.
        toString in class Object
        a string representation of the object.