hands-on-kotlin

Hands On Kotlin Presentation
Kotlin Yorkshire Meetup Group
20 April, 2017

Transcript

1. Hands-On Kotlin Yorkshire Kotlin Meetup 20th April, 2017
2. 0. Purpose of Evening Quick introduction to Kotlin An opportunity to get your hands dirty with a safety net
3. “ I hear and I forget. I see and I remember. I do and I understand. Confucius
4. 1. Data Types
5. Basic Data Types ◎ Similar to Java ◎ Numeric: ◦ Int, Long, Float, Double, Short, Byte ◎ Text ◦ String, Char ◎ Other ◦ Boolean ◦ Any - Root class in hierarchy ≈ Java Object ◦ Nothing In Kotlin, everything is an Object, there are no primitive types i.e. int, double, etc
6. Null Safety ◎ Kotlin attempts to eliminate Null Pointer Exceptions. ◎ By default, Kotlin variables/parameters will not accept Nulls ◎ Identify nullable values with a ‘?’ on datatype. ◦ E.g. Int? Or String? ◦ similar to Java Optional or Scala Option
7. Null Safety - Part 2 ◎ var a: String = "abc" a = null // compilation error, doesn’t allow null var b: String? = "abc" b = null // ok val l = b.length // error: 'b' can be null val l = b?.length // gives length or null // ‘Elvis’ operator val l = b?.length ?: -1 // gives length or -1 // Null checks can be chained together bob?.department?.head?.name val l = b!!.length // Throws NPE if b is null
8. Smart Casting ◎ Kotlin reduces the need to perform manual casting of variables. ◎ if (obj is String) { print(obj.length) // obj has been cast to String } ◎ // Automatically cast or right-side of && check if (x is String && x.length > 0) { print(x.length) } ◎ // When clauses when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) }
9. Safe Casting ◎ Manual casting is still permitted. val x: String = y as String // Will fail if y is null val x: String? = y as String? // Will handle null values ◎ Can improve handling using Safe Casting ‘as?’ to avoid ClassCastExceptions. ◎ // returns a String or null if y is not a String val x: String? = y as? String
10. 2. Control Flow
11. Control Flow - if condition ◎ if (condition) .. else .. ◎ Returns a value & so can be used as an expression. ◦ val totalValue = if (discountSelected) value * 0.85 else value ◎ Used as a ternary operator ◎ More complex expression blocks can be enclosed in braces {}. Last statement in each block provides the returned value.
12. Control Flow - when expression ◎ Similar to Java switch statement but even more powerful. Close to Scala pattern matching. ◎ Can select branch on: ◦ The value of an argument. ◦ The type of an argument ◦ An expression ◎ Default option is supplied by an ‘else’ condition. ◎ ‘when’ is an expression like ‘if’ and returns a result.
13. Control Flow - when examples ◎ Branch by Value when (x) { 1 -> print("x == 1") 2 -> print("x == 2") else -> { // Note the block print("x is neither 1 nor 2") } } ◎ Branch by Type val hasPrefix = when(x) { is String -> x.startsWith("prefix") else -> false } ◎ If.. Else If .. replacement when { x.isOdd() -> print("x is odd") x.isEven() -> print("x is even") else -> print("x is funny") }
14. Control Flow - For loops ◎ Iterate through items in a Collection (or anything that supplies an Iterator) ◎ Body of loop can be a single operation or a code block. ◎ Can supply numeric ranges ◦ for (i in 1..10) { println(i) } ◦ for (i in 1..4 step 2) print(i) ◦ for (i in 4 downTo 1 step 2) print(i)
15. Control Flow - While loops ◎ While loops are exactly like Java ◎ while (x > 0) { x-- } ◎ do { val y = readLine() } while (y != null)
16. 3. Functions
17. Function Definitions ◎ Functions can be declared inside or outside a Class. ◎ Local Functions can be declared inside another function. ◎ Parameters defined as name: type and separated by commas. ◎ Return types are optional (inferred) if function is a single expression but must be specified if function has a code block. ◎ ‘Unit’ return type is equivalent of Java ‘void’ but this is optional
18. Function Definitions - Parameters ◎ Parameters can be passed by name ◦ reformat(str, ◦ normalizeCase = true, ◦ divideByCamelHumps = false, ◦ wordSeparator = '_' ) ◎ Parameters can have default values ◦ fun read(b: Array<Byte>, off: Int = 0, len: Int = b.size()) { ◦ ... ◦ } ◎ Default values reduces the need to create overloaded methods.
19. Extension Functions ◎ Allows you to add functions to a Class without needing to subclass it. ◎ Can add functions to any Class even core Java classes. ◎ NB - Does not allow you to override existing methods. ◎ Provides a cleaner, more functional alternative to Java Utility classes with static methods.
20. Extension Functions (examples) ◎ fun String.toCamelCase() : String { return this.split(' ').map { it -> it.toLowerCase().capitalize() }.joinToString(separator = "") } ◎ fun MutableList<Int>.swap(index1: Int, index2: Int) { val tmp = this[index1] this[index1] = this[index2] this[index2] = tmp }
21. Lambda Expressions ◎ Lambda Expressions are blocks of code that optionally take parameters. ◎ Allow the code to be executed at a later point in time. ◎ Can be assigned to variables, passed as parameters and returned from functions.
22. Lambda Expressions - examples ◎ val sum: (Int, Int) -> Int = { x, y -> x + y } Simplified by removing inferred return type: val sum = { x: Int, y: Int -> x + y } ◎ Lambda can then be later invoked val total = sum(12, 19) ◎ If the lambda function only has 1 parameter it does not need to be declared but can be accessed by implicit name of ‘it’. orders.filter{ it.value > 100}
23. 4. Classes
24. Class Declaration ◎ Classes are defined with key word ‘class’ class Person constructor(firstName: String) {} ◎ Many classes can be defined in same file & name of the file does not need to match the class name. ◎ Each class has 1 primary constructor & may optionally have multiple secondary constructors. ◎ Instances of classes are created by calling constructor as if it was just a function. val customer = Customer("Joe Smith") ◎ By default, Kotlin classes are ‘final’. If you want a class to allows sub-classes it needs to be defined as ‘open’. open class Base(p: Int)
25. Class Parameters & Properties ◎ Constructors can have parameters defined just like a standard method. class Person constructor(firstName: String) {} ◎ These parameters are treated as properties if they are proceeded by ‘val’ or ‘var’. class Person(val firstName: String, val lastName: String, var age: Int) { // ...}
26. Data Classes ◎ Built in Support for Java Bean/ DTO classes. data class User(val name: String, val age: Int) ◎ Automatically generates the following methods at compile time: ◦ getter/setter for each property ◦ equals ◦ hashCode ◦ toString ◎ Dramatically reduces the amount of boilerplate code, even if that can be generated by the IDE.
27. Class Inheritance ◎ All classes extend the Any base class ◎ To explicitly extend another class need to pass parameters to parent constructor open class Person(firstName: String, surname: String) class Customer(accountNo: String,firstName: String, surname: String) : Person(firstName, surname) ◎ To override a method it must be declared as ‘open’ in the parent class & must be explicitly overriden. open class Base { open fun v() {} fun nv() {} } class Derived() : Base() { override fun v() {} }
28. Companion Objects ◎ Kotlin does not permit the creation of static methods on a Class. ◎ Can define Companion Objects within standard classes. class MyClass { companion object Factory { fun create(): MyClass = MyClass() } } val myInstance = MyClass.Factory.create()
29. Singleton Objects ◎ Singletons can be created using top-level object definitions (i.e. outside of a Class). This is similar to Scala ◎ object DataProviderManager { fun registerDataProvider(provider: DataProvider) { // ... } val allDataProviders: Collection<DataProvider> get() = // ... } DataProviderManager.registerDataProvider(...)
30. Give it a whirl Enough of the theory & syntax it’s time to do some coding.
31. Useful Links ◎ Kotlin Reference Documents: https://kotlinlang.org/docs/reference/ ◎ Kotlin Koans: https://kotlinlang.org/docs/tutorials/koans.html ◎ Online Koans: http://try.kotlinlang.org/ ◎ These slides: https://speakerdeck.com/andybowes/hands-on-kotlin