How does Kotlin implement working with primitive data types?

In Kotlin, primitive data types (Int, Double, Boolean, etc.) are represented as wrapper objects, but during compilation, they are optimized to JVM primitive types (e.g., int, double) for performance.

This means that from a developer's perspective, primitives behave like objects (for example, methods can be called on them), but under the hood, Kotlin uses JVM primitives to avoid overhead.

For example:

val x: Int = 42
val y: Int? = null // nullable type, here a wrapper object Integer is used

If the type is nullable, Kotlin always uses a wrapper object to be able to store null. If the type is not nullable, the compiler tries to use JVM primitives for optimization.

Thus, Kotlin combines the convenience of an object-oriented model with the efficiency of primitives.