Kotlin the Lighthouse Language

A brief introduction to Kotlin syntax as presented at the ironSource meetup on the 8th of August 2017

Transcript

1. The Lighthouse Language
2. Hi There! • Software Developer for three years ◦ Medical Research ◦ eCommerce • Android Developer at heart and profession • Software Developer at Gett • Co-Organizer KotlinTLV
4. So what is Kotlin? • Language developed by Jetbrains (IntelliJ) • Open sourced in 2011, Kotlin 1.0 on Feb 15th 2016 • 100% interoperable with Java • First class citizen language announcement in I/O 2017
5. Do we seriously need another language? Hear me out :)
6. Do we seriously need another language? • Costs nothing to adopt • Concise • Negligible Runtime overhead • Solves the Nullability Problem • Awesome and Modern
7. Concise? public class JavaPerson { private int age; private String name; private String address; public JavaPerson(int age, String name, String address) { this.age = age; this.name = name; this.address = address; } ...
8. Concise..? public void setAddress(String address) { this.address = address; } public void setAge(int age) { this.age = age; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public String getAddress() { return address; } public String getName() { return name; }
9. Yikes..? @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; JavaPerson that = (JavaPerson) o; if (age != that.age) return false; if (name != null ? !name.equals(that.name) : that.name != null) return false; return address != null ? address.equals(that.address) : that.address == null; } @Override public int hashCode() { int result = age; result = 31 * result + (name != null ? name.hashCode() : 0); result = 31 * result + (address != null ? address.hashCode() : 0); return result; } }
11. Good class KotlinPerson(var age: Int, var name: String, var address: String)
12. Better data class KotlinPerson(var age: Int, var name: String, var address: String)
13. Today • Variables • Functions • Extension Functions • Null Safety • Intro to Object-Oriented Programming
14. A few ground rules before we start • No static* • Everything that worked in Java will still work • No semicolons needed* * Except under adult supervision...
15. val/var name = "Hanan" // Compiler will complain val name: String = "Yarkoni" // public final String name = “Yarkoni” val name = "Yarkoni" // Type Inference
16. val/var var name: String = "Yarkoni" public String name = “Yarkoni” var name = "Yarkoni" // Type Inference name = "Hanan" // Compiler will be fine
17. Statically Typed Language var name = "Yarkoni" name = 5 // Compiler will complain name = "Hanan" // Compiler will be fine
18. 100% Java Interop val sb : StringBuilder = StringBuilder("Here") // Java StringBuilder* sb.append(" is some").append(" text!") // Java StringBuilder methods println(sb) // Wrapper for System.out.println
19. Questions?
20. Fun(ctions: Any)
21. Functions fun maximum(first: Int, second: Int): Int { if (first > second) { return first } else { return second } }
22. Functions - Default Values fun maximum(first: Int = 3, second: Int = 4): Int { if (first > second) { return first } else { return second } }
23. Default Values - Java 7 public int maximum() { return maximum(3, 4); } public int maximum(int first) { return maximum(first, 4); } public int maximumFirstDefault(int second) { return maximum(3, second); } public int maximum(int first, int second) { return first > second ? first : second; }
24. Function Calling // Regular call with parameters maximum(7, 6) // Named parameters maximum(first = 7, second = 6) maximum(second = 6, first = 7) maximum(second = 6)
25. Functions - Concise fun maximum(first: Int = 3, second: Int = 4): Int { if (first > second) { return first } else { return second } }
26. Functions - Concise fun maximum(first: Int = 3, second: Int = 4): Int = if (first > second) { first } else { second }
27. Functions - Concise fun maximum(first: Int = 3, second: Int = 4): Int = if (first > second) first else second
28. Functions - Concise fun maximum(first: Int = 3, second: Int = 4) = if (first > second) first else second
29. Extension Functions • Adopted from C# and Gosu • “Extension without Inheritance” • Basically “utils” functions
30. Extension Functions fun Int.maximum(other: Int): Int = if (this > other) this else other public static int maximum(int $receiver, int other) { if ($receiver > other) { return $receiver; } else { return other; } }
31. Extension Functions fun Int.maximum(other: Int): Int = if (this > other) this else other 7.maximum(6)
32. Other Function Features • Infix - no . needed • Operator - plus() is + • Inline (noinline) - Inlines a function • Functions within functions - double the fun!
33. Questions?
34. Safety?
35. Null Safety String amINull = "Not Null"; // Some time passes amINull = null; // Uh Oh // Some time passes amINull.equals("Null");
36. • An object reference in Java may or may not be null • Accessing that reference at compile is ok • Accessing that reference at runtime: ◦ May be ok ◦ May crash the system Nullability Problem Defined https://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare
37. • Java 8’s solution - Optional wrapper ◦ Grammar (.ofNullable, .of) ◦ I have to write down Optional?? ◦ I have to ask isPresent() and then get()???? • Kotlin’s solution - Built-in Nullable type Nullability Solution
38. Nullable Type val notNullString: String = "not null" val nullString: String? = null
39. Nullable Type - Usage nullString?.equals("Other string") // Safe nullString!!.equals("Other String") // Unsafe
40. Nullable Type - Chaining nullString?.toDoubleOrNull()?.compareTo(1) String nullString = ""; if (nullString != null) { if (Double.valueOf(nullString) != null) { return Double.valueOf(nullString).compareTo(1.0); } } return null;
41. Elvis Operator val result = nullString?.toDoubleOrNull()?.equals(1) ?: false
42. Questions?
43. Object-Oriented Programming
44. Object Oriented Programming - Rules • Interfaces may extend other interfaces • Classes my extend one other class • Classes may implement as many interfaces as needed
45. Kotlin Rules • Enums are called enum classes • Abstract is a soft keyword, but abstract exists • All objects must be fully initialized at creation • Java may extend Kotlin • Kotlin may extend Java
46. Open/Override class CodingExample { fun writeCode() = "I am writing code!" }
47. Open/Override open class CodingExample { open fun writeCode() = "I am writing code!" } class KotlinExample: CodingExample() { override fun writeCode() = "I am writing Kotlin Code" }
48. Open/Override open class CodingExample { open fun writeCode() = "I am writing code!" } class KotlinExample: CodingExample() { override fun writeCode() = "${super.writeCode()} I am writing Kotlin Code" }
49. Inheritance Example interface Person { fun fullName(): String } abstract class Bob implements Person { public abstract void fullName(); public String sayName() { return "My Name Is Bob"; } }
50. Inheritance Example interface Person { fun fullName(): String } abstract class Bob(open val firstName: String): Person { abstract fun fullName() fun sayName(): String = "My Name Is Bob" }
51. Inheritance Example abstract class Bob(open val firstName: String): Person { abstract fun fullName(): String //can be omitted fun sayName(): String = "My Name Is Bob" }
52. Inheritance Example - Continued class BobDylan: Bob("Bob") { // String interpolation is fun override fun fullName(): String = "${sayName()} Dylan" } abstract class Bob(open val firstName: String): Person { abstract fun fullName(): String //can be omitted fun sayName(): String = "My Name Is Bob" }
53. Inheritance Example - Continued class BobDole(override val firstName: String = "Bob", val lastName: String = "Dole"): Bob(firstName) { override fun fullName(): String = "I'm Bob Dole" } abstract class Bob(open val firstName: String): Person { abstract fun fullName(): String //can be omitted fun sayName(): String = "My Name Is Bob" }
54. Some Additions
55. Data Classes data class Elvis(val firstName: String = "Elvis", val lastName: String = "Presley") open data class Elvis(val firstName: String = "Elvis", val lastName: String = "Presley") Compiler will complain!
56. Sealed Classes sealed class Person(val firstName: String, val lastName: String) { class BobDylan: Person("Bob", "Dylan") class BobDole(val age: Int, fName: String, lName: String) : Person(fName, lName) }
57. Smart Casting fun translatePerson(person: Person): String = when (person) { is Person.BobDylan -> { "${person.firstName} ${person.lastName}" } is Person.BobDole -> { "${person.fName} ${person.lName}, of age ${person.age}" } }
58. Questions?
59. Thank You!