Understanding Kotlin's === Operator: Why Reference Equality Changes with Integer Values

Описание: Discover why Kotlin's reference equality operator `===` may return false for same object references with larger integers. Explore the effects of autoboxing and integer caching.
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Understanding Kotlin's === Operator: Why Reference Equality Changes with Integer Values

In Kotlin, the equality operator === is used to compare the reference of two objects. While this may seem straightforward at first, there can be unexpected behavior when it comes to autoboxing and certain integer values. In this post, we’ll explore why the operator can return false even when comparing what you would assume are identical references.

The Problem: Reference Equality in Kotlin

When you define and box integer values in Kotlin, you might encounter an interesting situation. For instance, consider the following pieces of code that illustrate how the === operator behaves differently for small and large integers:

[[See Video to Reveal this Text or Code Snippet]]

In this scenario, everything checks out perfectly—you would expect that any object with the same reference would yield a true statement. However, when the integer values change to 1000, the output for the boxed integers flips to false:

[[See Video to Reveal this Text or Code Snippet]]

This raises the question: Why does this happen?

The Solution: Autoboxing and Integer Caching

What is Autoboxing?

Autoboxing in Kotlin occurs when a primitive type (like Int) is converted into its corresponding wrapper type (like Int?). This process is implicit and is done automatically by the compiler. However, it also relies on certain optimization techniques to efficiently manage memory.

Integer Caching

Kotlin uses a caching mechanism for small integers (commonly between -128 to 127). When autoboxing these integers, if you box a value within this range, Kotlin will return the same object reference every time. This explains why we get true when comparing boxedA and anotherBoxedA if both are 100:

Small integers (-128 to 127): Reused cached reference.

Larger integers (128 and above): New object instances are created leading to different references.

Illustrated Example

For 100:

Both boxedA and anotherBoxedA reference the same cached integer object; therefore, boxedA === anotherBoxedA returns true.

For 1000:

Neither boxedA nor anotherBoxedA is pulled from the cache. They are treated as different instances, resulting in boxedA === anotherBoxedA returning false.

Conclusion

When working with Kotlin and the === operator for reference equality, it’s crucial to understand how autoboxing interacts with integer values. The caching of small integers leads to consistent behavior for values ranging from -128 to 127 but can introduce unexpected results for larger integers.

Kotlin handles many optimizations automatically for developers, making it essential to recognize these quirks. Armed with this knowledge, you can better anticipate how comparisons will behave in your code and prevent any surprises.

If you've found this explanation useful, feel free to share it with your fellow Kotlin developers or leave any questions in the comments below!