Kotlin and why you should love it #1

Introductory slides about Kotlin

1. KOTLIN AND WHY YOU SHOULD LOVE IT
2. WHAT IS KOTLIN? > Compatible with JVM (and moar) > Made by Jetbrains > More expressive > Safer > Functional > Uses Extension Functions > Highly interoperable
3. NULLABLE VS NONNULL > They are different types > Compile time error if you assign null to a non null variable > No more NullPointerException !
4. NULLABLE VS NONNULL > val name: String = null ❌ > val name: String = "Roberto" ✔ > val name: String? = null ✔
5. NULLABLE VS NONNULL (INTEROPERABILITY) > Calling Java from Kotlin ➡ Nullable > Unless it's annotated with @NonNull / @NotNull
6. SMART CAST void function(Object something) { if(something instanceof String) { String s = (String) something; println(s.substring(0, 3)) } }
7. SMART CAST fun function(something: Any) { if(something is String) { println(something.substring(0, 3)) } }
8. if you need explicit casting... pluto as Dog
9. DECLARING A CLASS class Person { ... }
10. DECLARING A CLASS class Person (name : String, surname : String?)
11. DECLARING A CLASS class Person (name : String, surname : String?) { init { //code common to every constructor } }
12. DECLARING A CLASS class Person (name : String, surname : String?) { constructor(name: String){ this(name, null) } init { //code common to every constructor } }
13. DECLARING A CLASS class Person (name : String, surname : String? = null) { init { //code common to every constructor } }
14. CREATING AN INSTANCE > val p = Person("Pippo") > val p2 = Person("Pippo", "Goofy")
15. CALLING A METHOD ON A NULLABLE TYPE val p = Person("Pippo") val fifthCharOfSurname: Char? = p.surname?.charAt(4) //this is nullable type fifthCharOfSurname?.doSomething() //executed only if not null
16. NULLABLE TYPES IN JAVA Person p = new Person("Pippo", null); String surname = p.getSurname(); if (surname != null) { Char fifthCharOfSurname = surname.charAt(4); if (fifthChar != null) { fifthCharOfSurname.doSomething(); } }
17. INHERITANCE class Person (name : String, surname : String) : Animal (name) { ... }
18. INTERFACES interface OnClickListener { fun onClick(v : View) }
19. INTERFACES AND DEFAULT METHODS interface OnClickListener { fun onClick(v : View) = println("I've been clicked") }
20. FUNCTIONS fun onCreate(savedInstanceState: Bundle?) { ... }
21. FUNCTIONS fun add(x : Int, y : Int) : Int { return x + y }
22. FUNCTIONS fun add(x : Int, y : Int) : Int = x + y
23. FUNCTIONS fun add(x : Int, y : Int) = x + y
24. FUNCTIONS fun add(x : Int = 1, y : Int = 2) = x + y
25. STRING TEMPLATES val s1 = "Hello, $who" val s2 = "Hello, ${person.name}"
26. VARIABLES > No implicit conversion: everything must be converted > Chars are not Ints, but we can convert them > Bitwise: | is or, & is and > Type can be inferred > Strings can be accessed as arrays
27. VARIABLES IMMUTABLE VS MUTABLE
28. VARIABLES val VS var
29. VARIABLES val a = 42 a = 43 // No can do! var b = "hello" b = "ciao" // Yeah can do var c : Context = this
30. VARIABLES class Person { var name : String = "" get() = field.toUppercase() set(value) { field = "Name: $value" } }
31. CONTROL FLOW
32. everything returns something
33. if val max = if (a > b) a else b
34. fun resultBasedOnCondition(condition: Boolean) { return if (condition) "A" else "B" }
35. when val type = when(fido) { is Dog -> "Dog" is Cat -> "Cat" else -> error("I only know about dogs and cats") }
36. DATA CLASSES data class Person (val name : String, val lastName : String)
37. DATA CLASSES GIVE US > equals() > hashCode() > copy()
38. EXTENSION METHODS //file Extension.kt fun Dog.bark() { ... } pluto.bark() fido.bark()
39. EXTENSION METHODS and how they are converted in Java static void bark(Dog dog) { ... } ExtensionKt.bark(pluto) ExtensionKt.bark(fido)
40. EXTENSION METHODS CAN BE DECLARED AS LOCAL FUNCTIONS
41. LAMBDAS
42. LAMBDAS ANONYMOUS FUNCTIONS FOR EVERYONE
43. JAVA view.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { doSomething(); } });
44. KOTLIN view.setOnClickListener(object : OnClickListener() { override fun onClick(v : View) { doSomething() } })
45. KOTLIN WITH LAMBDAS view.setOnClickListener({ view -> doSomething() })
46. KOTLIN WITH LAMBDAS view.setOnClickListener({ doSomething() })
47. KOTLIN WITH LAMBDAS view.setOnClickListener() { doSomething() }
48. KOTLIN WITH LAMBDAS view.setOnClickListener { doSomething() }
49. INLINE FUNCTIONS
50. INLINE FUNCTIONS they will be substituted by their code during compilation, instead of doing the real call to a function. inline fun <T> with(t: T, body: T.() -> Unit) { t.body() }
51. HOW TO MAKE AN INLINE FUNCTION inline fun ifIsLollipop(code : () -> Unit) { if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) { code() } }
52. THEN ifIsLollipop { window.setStatusBarColor(Color.BLACK) }
53. THANKS!