public FluentBackoff withInitialBackoff(Duration initialBackoff) {
    checkArgument(
        initialBackoff.isLongerThan(Duration.ZERO),
        "initialBackoff %s must be at least 1 millisecond",
        initialBackoff);
    return new FluentBackoff(
        exponent,
        initialBackoff,
        maxBackoff,
        maxCumulativeBackoff,
        maxRetries,
        throttledTimeCounter);
  }
@Test
  public void testInvalidInitialBackoff() {
    thrown.expect(IllegalArgumentException.class);
    thrown.expectMessage("initialBackoff PT0S must be at least 1 millisecond");
    defaultBackoff.withInitialBackoff(Duration.ZERO);
  }