Explain Kotlin code

What this Kotlin sample shows

null safety

note is a String?, so it may be null, and it is unless you pass one. note?.trim() calls trim only when there is a note, and ?: "no note" fills in when there isn't. Write note.trim() and the compiler stops: only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?.

extensions

fun Parcel.label() and fun Parcel.withExtra() add functions to Parcel from outside the class, so packingLabels can call it.label() as if it were a method. Inside them, code, grams, note and copy belong to the parcel they were called on.

data classes

data class writes equals, copy and destructuring for the three properties. withExtra(25) returns a 525 g copy and leaves the 500 g original as it was, original == original.copy() is true, and val (code, grams, note) = original gives BOX, 500 and null.

Good to know

• packingLabels turns a 500 g BOX without a note and a 100 g BAG whose note is fragile with two spaces on each side into BOX: 500 g (no note) and BAG: 100 g (fragile).
• withExtra changes only grams: the copy keeps the original's code and note, and the two parcels are not equal.
• An empty list of parcels gives an empty list of labels.

Watch out: ?: only steps in for null. A note of three spaces trims to an empty string, which isn't null, so a 20 g ENV parcel with that note gets the label ENV: 20 g (). note?.trim()?.ifBlank { null } ?: "no note" gives no note for it.