public FluentBackoff withExponent(double exponent) {
    checkArgument(exponent > 0, "exponent %s must be greater than 0", exponent);
    return new FluentBackoff(
        exponent,
        initialBackoff,
        maxBackoff,
        maxCumulativeBackoff,
        maxRetries,
        throttledTimeCounter);
  }
@Test
  public void testInvalidExponent() {
    thrown.expect(IllegalArgumentException.class);
    thrown.expectMessage("exponent -2.0 must be greater than 0");
    defaultBackoff.withExponent(-2.0);
  }