Mastering Kotlin’s copy() Function: Bytecode, Bitmasks & Hidden Pitfalls

Mastering Kotlin’s copy() Function: Bytecode, Bitmasks & Hidden Pitfalls

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TL;DR: Kotlin’s copy() is fast and elegant—but it performs a shallow copy under the hood, meaning nested mutable objects or collections can silently share state across instances if you aren't careful.

If you’ve built Android apps or worked with Kotlin, you’ve likely encountered data classes. They are the ultimate boilerplate cutters. And if you’re practicing immutable state management — like in MVI (Model-View-Intent) or Jetpack Compose — copy() is probably your most frequently used method.

But have you ever paused to think: What is the compiler actually doing when I call copy()? Is it creating a deep copy or a shallow copy? What's the performance cost?

Let’s unpack exactly how copy() works internally, look at the generated bytecode, explore advanced edge cases, and uncover the architectural pitfalls that catch even experienced developers off guard.

The Core Concept

At a high level, the copy() function is a compiler-generated method exclusive to Kotlin data classes. It allows you to create a new instance of an object by reusing unchanged properties from the original instance and selectively overriding only the ones you want to update.