CODEWITHSUNDEEP
genericskotlinlambda
Tarun Chawla
Tarun Chawla

Reputation: 538

Member reference not working for maxBy in Kotlin immutable list

Here is my code which is not working:

    val people = listOf(Person("Tarun", 28), Person("Shyam", 25), Person("Pushpraj", 27))
    people.maxBy { Person::age }

Error which am getting for above code:

Type parameter bound for R in inline fun <T, R : Comparable<R>> Iterable<T>.maxBy(selector: (T) -> R): T? is not satisfied: inferred type KProperty1<Person, Int> is not a subtype of Comparable<KProperty1<Person, Int>>

Working code:

    val people = listOf(Person("Tarun", 28), Person("Shyam", 25), Person("Pushpraj", 27))
    people.maxBy { it.age }

Not able to understand the issue here.

Upvotes: 1

Views: 502

Answers (1)

Neo
Neo

Reputation: 2039

TL;DR

people.maxBy(Person::age) would work (watch the brackets)

Method Signature

The maxBy function has following signature:

public inline fun <T, R : Comparable<R>> Iterable<T>.maxBy(selector: (T) -> R): T? {

If you write people.maxBy { Person::age }, it could be written as people.maxBy( { Person::age } ) which means you pass a lambda which returns another lambda (Supplier) which returns the INT age.

In other words: you pass (java Supplier) lambda inside other lambda instead of the actual (java Supplier) lambda. It looks confusing because you can remove normal brackets in Kotlin if the last argument in the method is a lambda.

Upvotes: 3

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