i notice that, when subclassing `MutableMap`, over...

# stdlib

[bbaldino]

i notice that, when subclassing MutableMap, overriding compute gives the signature override fun compute(key: T, remappingFunction: BiFunction<in T, in U?, out U?>): U?, but overriding computeIfAbsent gives override fun computeIfAbsent(key: T, mappingFunction: Function<in T, out U>): U (note the missing ? after U on the return type) but computeIfAbsent is documented as being able to return null. is this a bug? computeIfPresent also correctly uses U? as the return type.

[bbaldino]

@uli true, but i wondered why the others explicitly used U?

[bbaldino]

and i was running into an issue where, even though i had defined U as U : Any? in the class, explicitly returning null from that function was giving me an error

[uli]

The others are explicitly nullable because they can return null, even when U is non nullable. You can not make nullability go away for them

[bbaldino]

hmm, i got the impression computeIfAbsent was the same from the comment about the return value: @return the current (existing or computed) value associated with the specified key, or null if the computed value is null

[uli]

The others return a value U if the key is find, null otherwise.
ComputeIfAbsent returns a value U if the key is fond and a computed value U of not.

[uli]

That's the difference

[uli]

The docs just point out that the computed value U could be null if U happens to be a nullable type

[bbaldino]

gotcha...that's a subtle one. thanks @uli!

[uli]

Any time