Observer Pattern: Reacting to Changes

interview, best-practices

java java21 scala scala3 kotlin design-patterns observer-pattern

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Coming Soon — We're Still Hammering This One Out This post is on the roadmap and is being actively crafted. Think of it as a movie trailer: the good stuff is coming, you just caught us before the popcorn was ready. Check back soon — or bookmark the series guide to track all posts as they land.
⏳  Post in progress

What You'll Learn When This Post Is Ready

• How to implement a clean one-to-many notification system without creating spaghetti dependencies
• The difference between strong and weak references — and why getting it wrong means memory leaks that haunt you
• How modern reactive frameworks like RxJava and ZIO are sophisticated descendants of the Observer pattern
• How Scala's reactive streams and Kotlin's Flow build on the same foundational idea
• A real-world example: a stock price notification system with multiple subscriber types
• Side-by-side comparison across Java 21, Scala 3, and Kotlin with full test coverage

Interview Questions You Can Expect

• “Explain the Observer pattern and walk me through a simple implementation.”
• “What are the common pitfalls with observers, especially around memory? How do you fix them?”
• “How do modern reactive frameworks like RxJava or ZIO relate to the Observer pattern?”
• “What is the difference between observers and callbacks?”
• “How do you handle circular dependencies or notification storms with observers?”