Last week, I described a use-case for a custom Stream Collector. I received a intriguing comment on Twitter: There was a 'Not Found' error fetching URL: 'https://twitter.com/Miguucm/status/1389258245886390292' Hats off to you, Miguel! Your comment revealed a method I didn’t know! So I decided to investigate what the teeing() method is about. Returns a Collector that is a composite of two downstream collectors.

Java Stream’s Collectors methods fit most use-cases. They allow returning either a Collection or a scalar. For the former, you use one of the toXXX() method, for the latter, one of the reducing() one. Let’s imagine an e-commerce platform that implements a shopping cart. The cart is modeled as the following: This diagram might translate into the following (abridged) code: Product.

This week, I learned about a nifty 'new' feature of Optional that I want to share in this post. It’s available since Java 9, so its novelty is relative. Let’s start with the following sequence to compute the total price of an order: public BigDecimal getOrderPrice(Long orderId) { List<OrderLine> lines = orderRepository.findByOrderId(orderId); BigDecimal price = BigDecimal.ZERO; (1) for (OrderLine line : lines) { price = price.add(line.

When streams were added to Java 8, I wanted to jump on the bandwagon. I read blog posts, watched videos, attended workshops to understand what this was all about. After a while, I became comfortable enough…​ but I was unhappy with the implementation: while streams offer methods with a functional approach, legacy collections do not. When one wants to use Java capabilities on an existing collection, the later needs to be transformed to a stream, and then back again.

This is the 6th post in the Learning Clojure focus series. In general, one learns by comparing to what one already knows: I’m learning Clojure in that way. Coming from a Java background, I naturally want to use streaming features. So, what would be the Clojure counterparts of Java’s functions filter(), map(), etc.?