Open the ProductRepository interface and add one more method declaration on its getProductsByCategory method:

List<Product> getProductsByCategory(String category);

Open the implementation class InMemoryProductRepository and add an implementation for the previously declared method as follows:

public List<Product> getProductsByCategory(String category) {
  List<Product> productsByCategory = new ArrayList<Product>();
    
  for(Product product: listOfProducts) {
    if(category.equalsIgnoreCase(product.getCategory())){
      productsByCategory.add(product);
    }
  }
  
  return productsByCategory;
}

Similarly, open the ProductService interface and add one more method declaration on its getProductsByCategory method:

List<Product> getProductsByCategory(String category);

Open the service implementation class ProductServiceImpl and add an implementation...