Kotlin Brain Teasers

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Why does adding null + null produce a String?

Kotlin’s plus operator (+) can do different things—adding numbers, merging lists, joining strings, and so on—depending on the types of its inputs. Of these possible operations, it turns out that string concatenation is the only one that can accept a null value on both sides. That leaves Kotlin with no choice but to interpret null + null as an attempt to combine two nullable strings.

Anytime you use null in a string-building operation like this, Kotlin will replace it with the literal text "null". We’ve provided two null inputs, so we get two copies of the word "null", which are then stitched together just like any other pair of strings would be.

Operator Overloads

This is a fun puzzle, ...