Kotlin referential equality behavior on Int with values between -128 to 127

Question

I'm giving myself until 12:00AM to learn and get productive (hopefully) on kotlin.

Following https://kotlinlang.org/docs/kotlin-docs.pdf I tried these snippets on page 17. Could anyone please help me understand why === returns true if a value is between -128 to 127?

The following indeed prints false:

val a: Int = 10000
val boxedA: Int? = a            // Integer@445
val anotherBoxedA: Int? = a     // Integer@447 why?
print(boxedA === anotherBoxedA) // false

However changing a to any value between -128 to 127 always prints true:

val a: Int = -128
val boxedA: Int? = a            // Integer@445
val anotherBoxedA: Int? = a     // Integer@445 why?
print(boxedA === anotherBoxedA) // true!

It seems to me if Int value is outside the bounds of -128 to 127 (Java byte) kotlin creates a new object on assignment does making the reference not equal.

Answer 1

See the Java source code of Integer.valueOf() which is reponsible for boxing int values. The javadoc says:

This method will always cache values in the range -128 to 127

So boxed integers in that range are always the same object if they have the same numeric value.

In Kotlin, you should compare boxed Integers with == and not with ===.