8. Conventions [kotlin-workshop]

Part of https://github.com/jetBrains/kotlin-workshop.

Covers:
- Operator Overloading
- Other conventions: comparisons, in, iterator, etc.
- Destructuring Declarations
- Delegated Properties (incl. lazy example)

Transcript

1. Operator overloading & other conventions
2. plus operator fun Point.plus(other: Point): Point {
 return Point(x + other.x, y + other.y)
 } Point(1, 2) + Point(2, 3)
3. Arithmetic operations expression function name a + b plus a - b minus a * b times a / b div a % b mod
4. No restrictions on parameter type operator fun Point.times(scale: Int): Point {
 return Point(x * scale, y * scale)
 } Point(1, 2) * 3
5. Unary operations
6. Unary arithmetic operations expression function name +a unaryPlus -a unaryMinus !a not ++a, a++ inc —-a, a—- dec
7. Assignment operations
8. Conventions for lists val list = listOf(1, 2, 3)
 val newList = list + 2
 
 val mutableList = mutableListOf(1, 2, 3)
 mutableList += 4
9. Prefer val to var var list = listOf(1, 2, 3) list += 4 list = list + 4 new list is created:
10. Equality check
11. Comparisons "abc" < "def"
12. Comparisons symbol translated to a > b a.compareTo(b) > 0 a < b a.compareTo(b) < 0 a >= b a.compareTo(b) >= 0 a <= b a.compareTo(b) <= 0
13. Accessing elements by index: [] map[key]
 mutableMap[key] = newValue
14. The in convention if (key in map) { }
 if (element in list) { }
15. The rangeTo convention if (s in "abc".."def") { }
 for (i in 1..2) { }
 
 val oneTo100: IntRange = 1..100
 for (i in oneTo100) { }
16. The iterator convention operator fun CharSequence.iterator(): CharIterator for (c in "abc") { }
17. Destructuring declarations val (first, second) = pair
 for ((key, value) in map) { }
18. Destructuring in lambdas val map = mapOf(1 to "one", 2 to "two") map.mapValues { (key, value) -> "$key -> $value!" } map.mapValues { (_, value) -> "$value!" }
19. Lazy property val lazyValue: String by lazy {
 println("computed!")
 "Hello"
 }
 
 fun main(args: Array<String>) {
 println(lazyValue)
 println(lazyValue)
 } computed! Hello Hello
20. Delegated properties class C {
 var prop: Type by Delegate()
 }
21. Delegated properties class C {
 private val delegate = Delegate()
 var prop: Type
 set(value: Type) = delegate.setValue(..., value)
 get() = delegate.getValue(...)
 }
22. object Users : IdTable() { val name = varchar("name", length = 50).index() val age = integer("age") } class User(id: EntityID) : Entity(id) { var name: String by Users.name var age: Int by Users.age } Delegated properties in frameworks corresponds to a table in the database corresponds to a specific entry in this table columns values
23. object Users : IdTable() { val name = varchar("name", length = 50).index() val age = integer("age") } class User(id: EntityID) : Entity(id) { var name: String by Users.name var age: Int by Users.age } retrieves the value and updates it automatically Delegated properties in frameworks
24. Copyright © 2017 https://github.com/JetBrains/kotlin-workshop