Things that you might not know about Collections

1 minute read

Most functional programming has a great support for working with collections and so is Kotlin. If you are coding with Kotlin for sometimes, you might notice about map(), filter(), any() and forEach().

In Kotlin programming, we use collections extensively - List, MutableList and etc.

1. Am i a constructor?

constructor

Take a look at this piece of code

val list = List(5) { 'a' + it }
// output: [a, b, c, d, e]

This version of the List constructor has two parameters - the size of the List and a lambda that initializes each List element.

Remember that if a lambda is the last argument, it can be separated from the argument list.

Next, let’s look at MutableList. It can be initialized the same way.

val mutableList = MutableList(5, { it })
// [0, 1, 2, 3, 4]

Here is the interesting piece - both List() and MutableList() are not constructors in the Collections. They are functions. Their names begin with upper case intentionally, to make them look like constructor.

2. Filter(), the best option?

filter

filter() returns a group of elements satisfying the given predicate. What if you want the remaining of the group? It is filterNot() - that will do the work. In life, sometimes, you want us both - then how?

val list = listOf(-3, -1, 0, 1, 3)
val isPositive = { i: Int -> i > 0 }

// Option 1
val posOpt1 = list.filter(isPositive)
val negOpt1 = list.filterNot(isPositive)

// Option 2
val (posOpt2,negOpt2) = list.partition { it > 0 }

partition() produces a Pair object containing Lists.

3. Collection function on non comparable elements

collections

In List, we have functions like sum() or sorted() for a list of comparable elements. But what happens when we have a list of non-comparable elements? we have counterparts - sumBy() and sortedBy().

data class Product {
  val title: String,
  val price: Double
}

val products = listOf(
  Product("Pen", 2.5),
  Product("Ruler", 1.0)
)

products.sumByDouble { it.price } // 3.5
products.sortedByDescending { it.price }

We have two functions sumBy() and sumByDouble() to sum integer and double values respectively. sorted() and sortedBy() sort the collection in ascending order, while sortedDescending() and sortedByDescending() sort the collection in descending order.

Summary

Even though we only talk about List, operations like the above are also available for other collections like Set.

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