On this page
http://confluence.jetbrains.net/display/Kotlin/Function+literals
I find that this bit of code
``
fun max<T>(collection : Collection<out T>, less : (T, T) -> Boolean) : T? {
var max : T? = null
for (it in collection)
if (max == null || less(max, it))
max = it
return max
}
does not compile. Screenshot of what the IDE says is attached.
Also, what does the “out” in Collection<out T> do?
In line "if (max == null || less(max, it))" you pass variable max (type: T?) to function less that receives two not nullable arguments, so you should use .sure() function: "if (max == null || less(max.sure(), it))" . It is safe, because "less(max.sure(), it)" will be computed only if max != null.
About out you can read at http://confluence.jetbrains.net/display/Kotlin/Generics in section Variance.
In a few words, Collection<out T> means that we only can get T from the collection, but can’t put T into the collection.
Frostman wrote:
In line “if (max == null || less(max, it))” you pass variable max (type: T?) to function less that receives two not nullable arguments, so you should use .sure() function: “if (max == null || less(max.sure(), it))” . It is safe, because “less(max.sure(), it)” will be computed only if max != null.
Thanks! Since your reply, my plugin automatically updated, and now the IDE is much more precise, before runtime, about what the problem is. Screenshot attached.
And if I use “max.sure()” as you suggest, all the red in the IDE goes away.
Maybe I expect too much, but if max is null, the code after the OR is not going to execute. Which means in the post-OR part the IDE/compiler could relieve me of typing max.sure(). No?
Frostman wrote:
About out you can read at http://confluence.jetbrains.net/display/Kotlin/Generics in section Variance.
In a few words, Collection<out T> means that we only can get T from the collection, but can’t put T into the collection.
Thanks.
I find I have to read about generics really carefully and slowly, otherwise I get really confused. I find the finer points of Java generics somewhat odd, and I can’t figure out whether I have a mental block about all generics, or whether generics in Java are just weird. Either way, I find generics beyond trivial use cases, well, somewhat scary. It’s not so much that I cannot “memorize” the rules. It’s more about the need to know why the rule in the first place. Hope that makes sense.
The problem is that max is mutable, so we can't guarantee it won't change...Maybe I expect too much, but if max is null, the code after the OR is not going to execute. Which means in the post-OR part the IDE/compiler could relieve me of typing max.sure(). No?
