Difference between ByteArray and Array<Byte> in kotlin

Question

I don't understand why e.g. the java.security.MessageDigest.digest() method which is declared as returning byte[] in Java returns a ByteArray in Kotlin although Kotlin usually seems to call byte[] an Array<Byte>.

E.g. the following does not work:

fun main(args : Array<String>) {   val md = java.security.MessageDigest.getInstance("SHA")   if (md == null) throw NullPointerException()   val result : Array<Byte>? = md.digest()  } 

Type mismatch: inferred type is ByteArray? but Array<Byte>? was expected

Answer 1

Due to Java's limitations, Kotlin has 9 array types: Array<...> for arrays of references (in the JVM sense) and 8 specialized array types, i.e. IntArray, ByteArray etc.

https://kotlinlang.org/docs/reference/java-interop.html#java-arrays

The main reason for this distinction is performance: if we didn't specialize arrays it'd lead to a lot of boxing/unboxing and make arrays slow. This would be unacceptable because the only reason one might want to prefer arrays over collections is performance.