When building a map, which of these two guidelines...

# stdlib

[Klitos Kyriacou]

When building a map, which of these two guidelines do you think is better:
1. Use mapOf unless it has to be a hashmap, in which case use hashMapOf.
2. Use mapOf to document to your readers that entry order is important, or hashMapOf to document that order doesn't matter.

[Rob Elliot]

Use the more general unless there is a good reason to use the more specific. mapOf is the most general Map creation function.

It's a convenience that mapOf returns a LinkedHashMap, it's deliberately typed to return just a Map - if you need to document that you explicitly rely on insertion order you could use linkedMapOf.

(Incidentally while it's not in the common Kotlin type system, in practice on all three platforms, JS, JVM & Native, LinkedHashMap is a subtype of or alias for HashMap.)

[Klitos Kyriacou]

Thanks, Rob. Btw can you elaborate on your last sentence:

in practice on all three platforms, JS, JVM & Native, LinkedHashMap is a subtype or alias for HashMap.

I'm not sure what you mean.

[Rob Elliot]

Have a look at https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-linked-hash-map/

In kotlin common, kotlin.collections.LinkedHashMap : kotlin.collections.MutableMap, so it's not guaranteed to be a HashMap in all circumstances.

However, in practice there are only 3 Kotlin platforms, JVM, JS & Native, and on those:

• On the JVM kotlin.collections.LinkedHashMap is an alias for java.util.LinkedHashMap, and java.util.LinkedHashMap extends java.util.HashMap. On the JVM kotlin.collections.HashMap is an alias for java.util.HashMap
• On JS, class kotlin.collections.LinkedHashMap : kotlin.collections.HashMap
• On native, typealias kotlin.collections.LinkedHashMap = kotlin.collections.HashMap

[Klitos Kyriacou]

I see what you mean. We can use LinkedHashMap wherever a HashMap is required, but it does have a very slight overhead (i.e. the links) which may not be required, but in most circumstances the overhead can be ignored.