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Everyone’s always talking about how to write fast and efficient Java code… and it’s easy to write slow Java code. Or maybe you’re VIBE CODING and you need to identify badly written, non-performant code, so you can fix it, or tell your AI to fix it. In this lightning talk, we’ll show the best ways to write bad Java code! We’ll dive right into some code and We’ll also discuss how to fix it :)

Devoxx

A Java Champion walks through common Java anti-patterns and performance pitfalls, covering string concatenation in loops, boxing/unboxing overhead, regex compilation, data structure selection (ArrayList vs LinkedList vs HashSet), synchronization mistakes, and virtual thread pinning. The talk also addresses AI-generated code quality issues: hallucinated libraries, missing equals/hashCode implementations, exception-driven flow control, unbounded SQL queries, and over-indexing. Practical tips include using multiple AI models to cross-check generated code, crafting negative constraints in prompts, and always reviewing AI output before production use.

How to Write Bad Java Code by Pratik Patel