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It's the correct design only if we assume that the design space didn't allow for a different return type. Kotlin for example offers toIntOrNull (https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/to-...) as an alternative.

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A null doesn't contain any information about what went wrong and unless you religiously check your objects for null values at every turn you just turned a clear stack trace into a search for waldo at the international waldo impersonators meetup.

Note that in Kotlin the return type is `Int?`, not `Int`. You can't forget to check for null because the compiler enforces it.

To your first point: Another example in the design space would be Rust which works very similarly to Kotlin but returns more information in the failure case.

> Note that in Kotlin the return type is `Int?`, not `Int`.

How does that fare with unnecessary boxing of primitives?

If the function isn't total (as in: for every string there is an int) then the boxing is necessary, no?

Not if you have user-defined value types (soon coming to Java).

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