Hi all which is the more correct idiomatic way to declare th

Question

Hi all which is the more correct idiomatic way to declare this 1 ```private val registry MutableMap lt String MutableSet lt EventListener lt gt gt gt = hashMapOf ``` 2 ```private val registry = hashMapOf lt String MutableSet lt EventListener lt gt gt gt ```

Jacob · Answer

2 uses more type inference since it saves us from specifying the MutableMap type

Manuel Dossinger · Answer

In the first variant you have a `MutableMap` as you specified in the second it s a `HashMap` Slight difference If you do not really insist on having a hashMap I d say the most idiomatic is 3 ```private val registry = mutableMapOf lt gt ```

Tomasz Krakowiak · Answer

It s not public API so I don t think explicit type declaration is necessary Not much of an idiomatic problem Both statements have nearly exact same meaning The only difference between those two is that in 1 you explicitly mention that you don t use HashMap specific methods which is neglectable as it doesn t have any methods that are not present in MutableMap interface Whenever you worry about what s more idiomatic just try to translate it to English and think what expresses better what you meant to say 1 There s a private field registry of type MutableMap of event listeners and it s initialized with empty hash map 2 There s a private field registry initialized with empty hash map of event listeners So 2 is less verbose and carries at least all what I think that s need to be said

James Whitehead · Answer

Thanks everyone I was leaning towards number 2 as Tomasz said because it s not a public field and as Jacob mentioned for type inference which I felt was more idiomatic < Manuel Dossinger> when should one use hashMapOf over mutableMapOf

hho · Answer

I like the version of < Manuel Dossinger> best You shouldn t specify the implementation unless you really have to If at some point in the future a better mutableMap becomes available your code can take advantage of it However if you tie yourself to an implementation the possible advancements are pretty limited I checked the source and currently `mutableMapOf` returns a `LinkedHashMap` as opposed to a regular `HashMap` for `hashMapOf` So the tradeoff seems to be a little more memory usage but the insertion order is preserved for iteration

James Whitehead · Answer

Thanks < hho> much appreciated

Tomasz Krakowiak · Answer

Doesn t really matter what s the implementation behind `mutableMapOf` as long it s reasonable O log n `get` and `put` As < hho> said unless you require hash map specifically due to for example resources time constraints you should just say give me mutable map If for example you would need to preserve insertions order than you should say give me `LinkedHashMap` or even better give me map that preserves order but you should not use `mutableMapOf` despite it s implemented to return `LinkedHashMap` as there s not contract that `mutableMapOf` would return map that preserves order and it would be not clear for the person reading the code that your code depends on preserving order contract

hho · Answer

< Tomasz Krakowiak> yes there is such a contract The docs for `mutableMapOf` state that the iteration order is preserved

Tomasz Krakowiak · Answer

I think of coding as expressing my intentions that s the key to clean code

Tomasz Krakowiak · Answer

< hho> Ah okay My mistake ^^ Even though I would prefer explicitly `LinkedHashMap` as `mutableMapOf` is usually used to express give me any map and doesn t really explicitly implies dependency of the order preserving contract

Manuel Dossinger · Answer

< hho> thanks for jumping in I was writing an answer and got distracted by a call sweat smile

ephemient · Answer

mutableMapOf documents that order is preserved <https kotlinlang org api latest jvm stdlib kotlin collections mutable map of html|https kotlinlang org api latest jvm stdlib kotlin collections mutable map of html>