Unit 1 - Lesson 1 Kotlin basics.pdf

Lesson 1: Kotlin basics This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin v1.0...

Lesson 1: Kotlin basics This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin v1.0 1 About this lesson Lesson 1: Kotlin basics ○ Get started ○ Operators ○ Data types ○ Variables ○ Conditionals ○ Lists and arrays ○ Null safety ○ Summary This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 2  Get started This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 3 Open IntelliJ IDEA This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 4 Create a new project This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 5 Name the project This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 6 Open REPL (Read-Eval-Print-Loop) It may take a few moments before the Kotlin menu appears under Tools. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 7 Create a printHello() function Press Control+Enter (Command+Enter on a Mac) to execute. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 8 Online IDE https://play.kotlinlang.org/ Press ‘Run’ to execute the program This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 9  Operators This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 10 Operators Mathematical operators + - * / % Increment and decrement operators ++ -- < >= Comparison operators Assignment operator = Equality operators == != This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 11 Math operators with integers 1 + 1 => 2 53 - 3 => 50 50 / 10 => 5 9 % 3 => 0 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 12 Math operators with doubles 1.0 / 2.0 => 0.5 2.0 * 3.5 => 7.0 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 13 Math operators 1+1 1.0/2.0 ⇒ kotlin.Int = 2 ⇒ kotlin.Double = 0.5 53-3 2.0*3.5 ⇒ indicates output ⇒ kotlin.Int = 50 ⇒ kotlin.Double = 7.0 from your code. Result includes the 50/10 type (kotlin.Int). ⇒ kotlin.Int = 5 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 14 Numeric operator methods Kotlin keeps numbers as primitives, but lets you call methods on numbers as if they were objects. 2.times(3) ⇒ kotlin.Int = 6 3.5.plus(4) ⇒ kotlin.Double = 7.5 2.4.div(2) ⇒ kotlin.Double = 1.2 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 15  Data types This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 16 Integer types Type Bits Notes Long 64 From -263 to 263-1 Int 32 From -231 to 231-1 Short 16 From -32768 to 32767 Byte 8 From -128 to 127 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 17 Floating-point and other numeric types Type Bits Notes Double 64 16 - 17 significant digits Float 32 6 - 7 significant digits Char 16 16-bit Unicode character Boolean 8 True or false. Operations include: || - lazy disjunction, && - lazy conjunction, ! - negation This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 18 Operand types Results of operations keep the types of the operands 6*50 1/2 ⇒ kotlin.Int = 300 ⇒ kotlin.Int = 0 6.0*50.0 1.0*2.0 ⇒ kotlin.Double = 300.0 ⇒ kotlin.Double = 0.5 6.0*50 ⇒ kotlin.Double = 300.0 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 19 Type casting Assign an Int to a Byte val i: Int = 6 val b: Byte = i println(b) ⇒ error: type mismatch: inferred type is Int but Byte was expected Convert Int to Byte with casting val i: Int = 6 println(i.toByte()) ⇒ 6 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 20 Underscores for long numbers Use underscores to make long numeric constants more readable. val oneMillion = 1_000_000 val idNumber = 999_99_9999L val hexBytes = 0xFF_EC_DE_5E val bytes = 0b11010010_01101001_10010100_10010010 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 21 Strings Strings are any sequence of characters enclosed by double quotes. val s1 = "Hello world!" String literals can contain escape characters val s2 = "Hello world!\n" Or any arbitrary text delimited by a triple quote (""") val text = """ var bikes = 50 """ This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 22 String concatenation val numberOfDogs = 3 val numberOfCats = 2 "I have $numberOfDogs dogs" + " and $numberOfCats cats" => I have 3 dogs and 2 cats This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 23 String templates A template expression starts with a dollar sign ($) and can be a simple value: val i = 10 println("i = $i") => i = 10 Or an expression inside curly braces: val s = "abc" println("$s.length is ${s.length}") => abc.length is 3 This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 24 String template expressions val numberOfShirts = 10 val numberOfPants = 5 "I have ${numberOfShirts + numberOfPants} items of clothing" => I have 15 items of clothing This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 25  Variables This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 26 Variables Powerful type inference Let the compiler infer the type You can explicitly declare the type if needed Mutable and immutable variables Immutability not enforced, but recommended Kotlin is a statically-typed language. The type is resolved at compile time and never changes. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 27 Specifying the variable type Colon Notation var width: Int = 12 var length: Double = 2.5 Important: Once a type has been assigned by you or the compiler, you can't change the type or you get an error. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 28 Mutable and immutable variables Mutable (Changeable) var score = 10 Immutable (Unchangeable) val name = "Jennifer" Although not strictly enforced, using immutable variables is recommended in most cases. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 29 var and val var count = 1 count = 2 val size = 1 size = 2 => Error: val cannot be reassigned This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 30  Conditionals This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 31 Control flow Kotlin features several ways to implement conditional logic: If/Else statements When statements For loops While loops This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 32 if/else statements val numberOfCups = 30 val numberOfPlates = 50 if (numberOfCups > numberOfPlates) { println("Too many cups!") } else { println("Not enough cups!") } => Not enough cups! This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 33 if statement with multiple cases val guests = 30 if (guests == 0) { println("No guests") } else if (guests < 20) { println("Small group of people") } else { println("Large group of people!") } ⇒ Large group of people! This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 34 Ranges Data type containing a span of comparable values (e.g., integers from 1 to 100 inclusive) Ranges are bounded Objects within a range can be mutable or immutable This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 35 Ranges in if/else statements val numberOfStudents = 50 if (numberOfStudents in 1..100) { println(numberOfStudents) } => 50 Note: There are no spaces around the "range to" operator (1..100) This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 36 when statement when (results) { 0 -> println("No results") in 1..39 -> println("Got results!") else -> println("That's a lot of results!") } ⇒ That's a lot of results! As well as a when statement, you can also define a when expression that provides a return value. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 37 for loops val pets = arrayOf("dog", "cat", "canary") for (element in pets) { print(element + " ") } ⇒ dog cat canary You don’t need to define an iterator variable and increment it for each pass. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 38 for loops: elements and indexes for ((index, element) in pets.withIndex()) { println("Item at $index is $element\n") } ⇒ Item at 0 is dog Item at 1 is cat Item at 2 is canary This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 39 for loops: step sizes and ranges for (i in 1..5) print(i) ⇒ 12345 for (i in 5 downTo 1) print(i) ⇒ 54321 for (i in 3..6 step 2) print(i) ⇒ 35 for (i in 'd'..'g') print (i) ⇒ defg This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 40 while loops var bicycles = 0 while (bicycles < 50) { bicycles++ } println("$bicycles bicycles in the bicycle rack\n") ⇒ 50 bicycles in the bicycle rack do { bicycles-- } while (bicycles > 50) println("$bicycles bicycles in the bicycle rack\n") ⇒ 49 bicycles in the bicycle rack This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 41 repeat loops repeat(2) { print("Hello!") } ⇒ Hello!Hello! This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 42 Lists and arrays This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 43 Lists Lists are ordered collections of elements List elements can be accessed programmatically through their indices Elements can occur more than once in a list An example of a list is a sentence: it's a group of words, their order is important, and they can repeat. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 44 Immutable list using listOf() Declare a list using listOf() and print it out. val instruments = listOf("trumpet", "piano", "violin") println(instruments) ⇒ [trumpet, piano, violin] This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 45 Mutable list using mutableListOf() Lists can be changed using mutableListOf() val myList = mutableListOf("trumpet", "piano", "violin") myList.remove("violin") ⇒ kotlin.Boolean = true With a list defined with val, you can't change which list the variable refers to, but you can still change the contents of the list. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 46 Arrays Arrays store multiple items Array elements can be accessed programmatically through their indices Array elements are mutable Array size is fixed This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 47 Array using arrayOf() An array of strings can be created using arrayOf() val pets = arrayOf("dog", "cat", "canary") println(java.util.Arrays.toString(pets)) ⇒ [dog, cat, canary] With an array defined with val, you can't change which array the variable refers to, but you can still change the contents of the array. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 48 Arrays with mixed or single types An array can contain different types. val mix = arrayOf("hats", 2) An array can also contain just one type (integers in this case). val numbers = intArrayOf(1, 2, 3) This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 49 Combining arrays Use the + operator. val numbers = intArrayOf(1,2,3) val numbers2 = intArrayOf(4,5,6) val combined = numbers2 + numbers println(Arrays.toString(combined)) => [4, 5, 6, 1, 2, 3] This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 50  Null safety This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 51 Null safety In Kotlin, variables cannot be null by default You can explicitly assign a variable to null using the safe call operator Allow null-pointer exceptions using the !! operator You can test for null using the elvis (?:) operator This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 52 Variables cannot be null In Kotlin, null variables are not allowed by default. Declare an Int and assign null to it. var numberOfBooks: Int = null ⇒ error: null can not be a value of a non-null type Int This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 53 Safe call operator The safe call operator (?), after the type indicates that a variable can be null. Declare an Int? as nullable var numberOfBooks: Int? = null In general, do not set a variable to null as it may have unwanted consequences. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 54 Testing for null Check whether the numberOfBooks variable is not null. Then decrement that variable. var numberOfBooks = 6 if (numberOfBooks != null) { numberOfBooks = numberOfBooks.dec() } Now look at the Kotlin way of writing it, using the safe call operator. var numberOfBooks = 6 numberOfBooks = numberOfBooks?.dec() This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 55 The !! operator If you’re certain a variable won’t be null, use !! to force the variable into a non- null type. Then you can call methods/properties on it. val len = s!!.length throws NullPointerException if s is null Warning: Because !! will throw an exception, it should only be used when it would be exceptional to hold a null value. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 56 Elvis operator Chain null tests with the ?: operator. numberOfBooks = numberOfBooks?.dec() ?: 0 The ?: operator is sometimes called the "Elvis operator," because it's like a smiley on its side with a pompadour hairstyle, like Elvis Presley styled his hair. This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 57  Summary This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 58 Summary In Lesson 1, you learned how to: Create an IntelliJ IDEA project, opening REPL, and execute a function Use operators and numeric operator methods Use data types, type casting, strings, and string templates Use variables and type inference, and mutable and immutable variables Use conditionals, control flow, and looping structures Use lists and arrays Use Kotlin's null safety features This work is licensed under the Apache 2 license. Apache 2 license Android Development with Kotlin 59