Long/Int in scala to hex string

val bigNum: Long   = 13690566117625L
val bigHex: String = f"$bigNum%X"

Use %X to get uppercase hex letters and %x if you want lowercase.

You're looking for RichLong.toHexString:

scala> 13690566117625L.toHexString
res0: String = c73955488f9

And the uppercase variant:

scala> 13690566117625L.toHexString.toUpperCase
res1: String = C73955488F9

Edit

This also available for Int via RichInt.toHexString:

scala> 42.toHexString
res4: String = 2a