OpenValue Düsseldorf

Also this time, we are giving away 5 tickets to the JCON Conference 2026 to all participants of the meetup. It will take place from April 21 to April 23 in Cologne. You'll find more information on the JCON website.

### 🗓️ Agenda

• 17:30 – Doors open / Walk-in
• 18:00 – First Talk
• 18:45 – Break
• 19:00 – Second Talk
• 19:45 – Drinks & Discussions

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### 🎤 Talks

#### Scaling Integration Tests: Parallel Spring Tests with JUnit & Testcontainers

by Marvin Rensing
Integration tests can quickly become slow, especially when every test starts its own Spring Boot context and Testcontainers instances. In this session, we’ll have a look at how to run such tests in parallel using JUnit 5, Testcontainers and WireMock. We’ll talk about what needs to be in place for parallel execution to work reliably: how to avoid flaky tests, keep them independent, and reuse containers and application contexts where possible. We’ll also look at how JUnit extensions can help set up isolated environments for each test, without dragging our test suite down. Finally, we’ll share some lessons learned from real projects. What worked, what didn’t, and when the extra effort for parallel execution actually pays off.

#### A billion dollar problem: Proper null handling in Java

by Benedikt Loup
The null reference is often regarded as one of the most consequential design flaws in software engineering history. Tony Hoare, the inventor of the null reference, famously stated his invention as billion dollar problem, when taking into consideration the amount of hours spent into debugging such errors. In Java, it undermines the otherwise strong type safety of the language, since any object reference can potentially be null - leading to runtime errors and system crashes which can be quite expensive. This talk explores the origins and implications of the null problem and examines various strategies to prevent the famous NullPointerException. Beyond traditional null checks which sometimes make the code harder to read, it discusses modern alternatives such as Optionals, structured exception handling, and annotation-based approaches like JSpecify and shows the pros and cons of these approaches. Furthermore, a preview feature introducing "Null-Restricted Value Class Types" is being investigated to determine how the aforementioned issued can be resolved with it or not. The goal is to demonstrate how a deliberate and consistent handling of null values can significantly improve the reliability, readability, and maintainability of Java applications. After the session you’ll be able to fight NullPointerExceptions in your projects without messing up readability and complexity.

We're looking forward to another great evening of learning, sharing, and networking—don’t miss out!

See you on Feruary 19th! 👋