Is there a good explanation somewhere of why `Map`...

# announcements

[Nir]

Is there a good explanation somewhere of why Map does not inherit from Collection ?

[Zach Klippenstein (he/him) [MOD]]

It doesn't in Java, so it can't in kotlin.

[Zach Klippenstein (he/him) [MOD]]

Why it doesn't in Java is another question, and I don't know the answer.

[Nir]

ah that's a real shame

[Nir]

i thought things like the way MutableMap inherits from Map is already different from Java though?

[stephanmg]

Why should it inherit from Collection?

[Nir]

https://stackoverflow.com/questions/2651819/why-doesnt-java-map-extend-collection#:~:text=Each%20Collection%20stores%20a%20single,are%20incompatible%20for%20Map%20interface. yikes

[Nir]

@stephanmg i'd have to look more carefully at the collection interface to see what's specific to collection. It is clearly iterable though.

[stephanmg]

collection extends Iterable

[stephanmg]

Collection

[Nir]

I understand that

[Nir]

I expected Map to extend collection, the same as List

[Nir]

but it doesn't even extend Iterable

[stephanmg]

well since you posted the StackOverflow question

[stephanmg]

don’t you agree with the accepted answer?

[Nir]

Uh, no, it's pretty terrible? and dated

[Nir]

Simply the fact that there's an obvious implementation of iteration, and map, should tell you that Map should at least be Iterable

[Zach Klippenstein (he/him) [MOD]]

I could certainly see a counter argument that a map can be seen as a collection of entries, but I guess that is a secondary aspect of it that java didn't want to highlight by encoding it in the type system?

[Nir]

Sure, it's clear that java didn't want to, the question is why?

[Zach Klippenstein (he/him) [MOD]]

Map and MutableMap work because they're actually the same JVM type.

[Nir]

So, Collection has a size/length attribute, right

[Nir]

Maps do as well

[stephanmg]

@Nir I agree with you

[stephanmg]

but I presume by the time it was, as they said, a design decision, to differentiate…

[stephanmg]

I do not know why.

[Nir]

https://en.wikipedia.org/wiki/Collection_(abstract_data_type)

[stephanmg]

If you read carefully they (Java FAQ) write: “we feel”.

[stephanmg]

so this is not set in stone in general

[frogger]

You’re probably looking for Collection#entrySet which is a collection.

[Nir]

uh no, I'm not 🙂

[stephanmg]

@Nir I understand your conceptional problem

[Nir]

What's annoying about all this is that currently if you want to implement something "map like", you now have to do it 3 times

[stephanmg]

that’s true

[Nir]

once for iterable, once for sequence, and once for map

[Nir]

sorry, for Map 🙂

[Nir]

all that Collection interface adds is:
• size
• contains
• containsAll
• isEmpty
• Iterator

[Nir]

All of which make perfect sense

[Nir]

for a Map

[Nir]

I wonder how reasonable it would be to write your own ReadOnlyMap which inherited from both Map and Iterable

[Nir]

Map and Collection I mean

[stephanmg]

@Nir: you are very certain about your assertions

[stephanmg]

😄

[stephanmg]

at least the original developers would disagree (Java)

[stephanmg]

but IMHO it makes sense

[stephanmg]

but this is just my point of view

[Nir]

It's possible they would, I can't say, and maybe they actually had good reasons, but that SO answer does not show good reasons

[Nir]

The developers of basically all other languages would disagree :-) associative maps always conform to the collection/container/iterable interface

[Nir]

In every other language I have used

[Nir]

Python, C++, Rust, to name a few

[stephanmg]

Yeah

[stephanmg]

as I said they write “feel”

[stephanmg]

and this is like 15 years ago probably

[stephanmg]

so maybe today they would “feel” other way

[stephanmg]

as you maybe

[stephanmg]

🙂

[Nir]

heh maybe. Most of that quote is actually discussing alternative things that very obviously make no sense, like having Collection inherit from Map.

[Nir]

There's only like 1 sentence about why Map inheriting Collection isnt' good

[Zach Klippenstein (he/him) [MOD]]

I'm guessing there are historical reasons for java - eg before generics were added in (1.5?) it may not have been clear which parts of the interface applied to keys, values, or entries.

[Nir]

yeah, possibly there's more API in collections in Java than in Kotlin.

[Nir]

From what I can glean from the answer, it seems like it's at least partly that they just thought super heavily in terms of mutation

[Nir]

if you don't have something like map, then yes, you might question what the purpose of looking at associative containers that way is

[Nir]

instead push people to be explicit about what they want to iterate over 🤷

[Nir]

On the off chance anyone cares: I had my ImmutableMap class implement both Map and Collection, trivially implemented the methods in question, everything seems to be fine

[Nir]

There's actually no ambiguity errors,.almost all of iterable is extension methods, so the member functions get priority

[Nir]

Eg iterable map vs Map.map

[stephanmg]

Nir, did you discuss with Java folks?

[stephanmg]

Did they offer an answer?

[Nir]

No. where should I go to discuss with Java folks?

[stephanmg]

I do not know.

[stephanmg]

Just a suggestion

[Nir]

i could see if there's a java slack or something. I feel like the odds of getting a good answer are low though.

[stephanmg]

well @Nir if you do not try, you never know 😄

[stephanmg]

I presume it will be a fruitful discussion

[stephanmg]

did you consider using Usenet?

[stephanmg]

comp.lang.java?