Test case with ArgumentCaptor

   @Test
   public void it_should_save_number_when_flag_is_true() {
        //Given
        ArgumentCaptor<Product> productArgumentCaptor = ArgumentCaptor.forClass(Product.class);
        Product product = Product.builder().isFlag(true).build();

        //When
        productService.save(product);

        //Then
        verify(productRepository).save(productArgumentCaptor.capture());

        final Product capturedProduct = productArgumentCaptor.getValue();
        assertThat(capturedProduct.getIsFlag()).isTrue();
    }