Title: Bulletproofing your foot for Kotlin
Speakers: Enrique Zamudio
Slides (from the Devoxx version of this talk)
For more blog posts, see the DevNexus 2018 live blogging table of contents
About Kotlin
- Created by JetBrains (makers of IntelliJ)
- Static typing
- JVM language
- Can run on front end or back end
- Good for Android
Data classes
data class Person(val name:String val birthDate:Date)
- No need to write getters/setters.
- toString() and equals() automatically added.
- However hashCode() is not automatically added.
Destructuring
val p = Person("John", now)
val (n, f) = p
- Like Scala
- Stores each field in a variable
- Similarly can do for ((k,v) in map) when iterating through a map
Functions
fun foo() {}
Takes no params, calls a function and returns Unit (void)
fun bar(f:() -> (Unit)) = f())
Method reference like in Java. Since a top level function, nothing before the ::
bar(::foo)
Passing a lambda directly to a function:
bar ( { println() })
Runs on Java 7
String interpolation
println "Name: $n"
Works like in Groovy
Type inference
Immutable (final) String:
val p = "hello"
Mutable:
var q = "hi"
Recommend specifying type for method so faster.
Smart casts
val p:Any = "string"
if p is String) {
println(p.toUppercase())
}
- Any is a root class
- Once you check the type, you can use it rather than having to cast.
- Can still write explicit cast println((p as String).toUpperCase()). Stupid cast?
Typesafe null
var p:String? = "hello"
p = null
if (p != null) {foo(x)}
- Want to catch errors at compile time
- If just write var, can’t assign null.
- Have to explicitly say it is ok by making it an “optional string”
- Cannot pass an optional string to a method that takes a regular string [cool!]
- Write guard to check if not null. That is a smart cast to turn String? into String so can call method that takes String
- ?. returns null if it is null vs a runtime exception. So safe way is to write s?.length
- The elvis operator ?: from Groovy is also supported.
- BEWARE: There’s also a kill switch. Writing foo(p!!) you are forcing Kotlin to pass the String? into String without the check. It is a way to say you know what you are doing. Then you get a runtime error if it is null instead of a compiler error.
Operator overloading
class Foo(val x:Int) {
operator fun plus(o:Foo) = Foo(o.x=x)
}
- Can only overload simple operators – math operators and index (square square brackets)
- BEWARE: only way to tell if can use overloading is to look at the class code (or release notes). It’s not in the docs.
Extension methods
fun Person.debug() { println("") {
- Can add methods to a class even if don’t have the source code.
- BEWARE: don’t use extension operators. Someone looking at the class won’t know they are there. Not obvious where to look for the code.
Non local returns
- BEWARE: do not use.
- Allows writing a return within a closure. In Groovy, it would return from the closure. In Kotlin, it returns from the whole method.
- Can write return@forEach to return from just the closure. But yech.
My take
This was my first exposure to Kotlin. Some of the features are really cool!
