public DefaultIssue setGap(@Nullable Double d) {
    Preconditions.checkArgument(d == null || d >= 0, "Gap must be greater than or equal 0 (got %s)", d);
    this.gap = d;
    return this;
  }
@Test
  void setGap_whenGapIsNegative_shouldThrowException() {
    Double anyNegativeValue = -1.0;
    assertThatThrownBy(() -> issue.setGap(anyNegativeValue))
      .isInstanceOf(IllegalArgumentException.class)
      .hasMessage(String.format("Gap must be greater than or equal 0 (got %s)", anyNegativeValue));
  }