@ElementTypesAreNonnullByDefault
public abstract class Ticker {
  /** Constructor for use by subclasses. */
  protected Ticker() {}

  /** Returns the number of nanoseconds elapsed since this ticker's fixed point of reference. */
  public abstract long read();

  /**
   * A ticker that reads the current time using {@link System#nanoTime}.
   *
   * @since 10.0
   */
  public static Ticker systemTicker() {
    return SYSTEM_TICKER;
  }

   *
   * @since 15.0
   */
  public static Stopwatch createStarted(Ticker ticker) {
    return new Stopwatch(ticker).start();
  }

  Stopwatch() {
    this.ticker = Ticker.systemTicker();
  }

  Stopwatch(Ticker ticker) {
    this.ticker = checkNotNull(ticker, "ticker");
  }

  /**

  public void testAutoRefresh() {
    FakeTicker ticker = new FakeTicker();
    IncrementingLoader loader = incrementingLoader();
    LoadingCache<Integer, Integer> cache =
        CacheBuilder.newBuilder()
            .refreshAfterWrite(3, MILLISECONDS)
            .expireAfterWrite(6, MILLISECONDS)
            .lenientParsing()
            .ticker(ticker)
            .build(loader);
    int expectedLoads = 0;

@GwtIncompatible
public class StopwatchJavaTimeTest extends TestCase {
  private final FakeTicker ticker = new FakeTicker();
  private final Stopwatch stopwatch = new Stopwatch(ticker);

  public void testElapsed_duration() {
    stopwatch.start();
    ticker.advance(999999);
    assertEquals(Duration.ofNanos(999999), stopwatch.elapsed());
    ticker.advance(1);
    assertEquals(Duration.ofMillis(1), stopwatch.elapsed());
  }

import com.google.common.annotations.J2ktIncompatible;
import com.google.common.base.Ticker;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.time.Duration;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * A Ticker whose value can be advanced programmatically in test.
 *
 * <p>The ticker can be configured so that the time is incremented whenever {@link #read} is called: