Returning default list if the list is empty using java 8 Streams?

You can try this:

List<Product> recommendedProducts 
        = this.newProducts
              .stream()
              .filter(isAvailable)
              .collect(Collectors.collectingAndThen(Collectors.toList(), list -> list.isEmpty() ? DEFAULT_PRODUCTS : list));

While you could achieve your goal using Optional, I would still opt for plain old ternary operator.

In this particular case it makes much more sense and improves readability:

return recommendedProducts.isEmpty() ? DEFAULT_PRODUCTS : recommendedProducts;