Kotlin Data Types

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Unit:

• Indicates that no meaningful value is returned.
• Is the default inferred return type
• Its purpose is the same as void in Java, but it is a proper type, so it can be passed as a generic argument, etc.
• It wasn’t named Void because of the potential confusion between Void and Nothing.

Nothing: - Indicates that this function will never return (always throws an exception) - When used as a return type, it doesn’t affect inferred types - Is the subtype of all other types.

Collections: The collection interface supports only read-only methods while the MutableCollection interface supports read/write methods.

• The method listOf<T>() is used to create a simple, read-only List.
• mutableListOf<T>() creates a MutableList

There are other similar collection literals, such as ||| |–|–| |listOf()|mutableListOf()| |setOf()|mutableSetOf()| |mapOf()|mutableMapOf()|

However, read-only does not mean that they are fully immutable. The underlying object can still be changed.

fun funWithLists(){
    val m = mutableListOf(1,2,3)
    val list: List<Int> = m

    println(list) // [1, 2, 3]

    m.add(4)

    println(list) // [1, 2, 3, 4]

    list.add(5) // ERROR!

}

Sequences

• Ordered collection of elements, potentially unbounded in size
• Values are evalued lazily
• Similar to streams in Java 8

However, if you try to evaluate an infinite list all at once, you will get an OutOfMemoryError, so watch out!

Other built in methods:

• map
• flatMap
• filterIsInstance
• onEach
• partition
• toList
• toMutableList

Resources: Kotlin Types: Exposed The Road to Kotlintown: Part 1 The Road to Kotlintown: Part 2 Kotlin in Action by Dmitry Jemerov and Svetlana Isakova

Koans (let’s start here): https://play.kotlinlang.org/byExample/01_introduction/04_Null%20Safety