public void setExternalTaskQueueCapacity(int externalTaskQueueCapacity) {
        this.externalTaskQueueCapacity = checkPositive(externalTaskQueueCapacity, "externalTaskQueueCapacity");
    }
@Test
    public void test_setExternalTaskQueueCapacity_whenNegative() {
        ReactorBuilder builder = newBuilder();
        assertThrows(IllegalArgumentException.class, () -> builder.setExternalTaskQueueCapacity(-1));
    }