// purchase product and return relevant product recommendations
public List<Product> purchase(Customer customer, Product product) {
purchasesCache.put(customer.id, product.id);
ProductAssociativityGraph graph = productAssociativityGraphMap.get(product.category);
Vertex v = Vertex.create(product.id);
List<Vertex> associations = graph.getProductAssociations(v);
int recommendSize = Math.min(associations.size(), maxNumRecommendations);
return associations
.stream()
.map(vertex -> productCache.get(vertex.productId))
.limit(recommendSize)
.collect(Collectors.toList());
}
