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Lesson 1:

Kotlin basics

This work is licensed under the Apache 2 license.

Android Development with Kotlin v1.0

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About this lesson

Lesson 1: Kotlin basics

Get started
Operators
Data types
Variables
Conditionals
Lists and arrays
Null safety
Summary

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Android Development with Kotlin

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Get started

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Android Development with Kotlin

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Open IntelliJ IDEA

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Android Development with Kotlin

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Create a new project

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Name the project

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Android Development with Kotlin

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Open REPL (Read-Eval-Print-Loop)

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It may take a few moments before the Kotlin menu appears under Tools.

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Create a printHello() function

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Press Control+Enter (Command+Enter on a Mac) to execute.

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Operators

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Operators

Mathematical operators

Assignment operator

Equality operators

Increment and decrement operators

Comparison operators

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- * / %

=

== !=

++ --

< <= > >=

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Math operators with integers

1 + 1 =>

2

53 - 3 =>

50

50 / 10 =>

5

9 % 3 =>

0

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Math operators with doubles

1.0 / 2.0 =>

0.5

2.0 * 3.5 =>

7.0

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Math operators

1+1

⇒ kotlin.Int = 2

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⇒ indicates output from your code.

Result includes the type (kotlin.Int).

2.0*3.5

⇒ kotlin.Double = 7.0

1.0/2.0

⇒ kotlin.Double = 0.5

50/10

⇒ kotlin.Int = 5

53-3

⇒ kotlin.Int = 50

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Numeric operator methods

Kotlin keeps numbers as primitives, but lets you call methods on numbers as if they were objects.

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2.4.div(2)

⇒ kotlin.Double = 1.2

3.5.plus(4)

⇒ kotlin.Double = 7.5

2.times(3)

⇒ kotlin.Int = 6

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Data types

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Integer types

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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

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Floating-point and other numeric types

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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

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Operand types

Results of operations keep the types of the operands

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6.0*50

⇒ kotlin.Double = 300.0

6.0*50.0

⇒ kotlin.Double = 300.0

6*50

⇒ kotlin.Int = 300

1/2

⇒ kotlin.Int = 0

1.0*2.0

⇒ kotlin.Double = 0.5

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Type casting

Assign an Int to a Byte

Convert Int to Byte with casting

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val i: Int = 6

println(i.toByte())

⇒ 6

val i: Int = 6

val b: Byte = i

⇒ error: type mismatch: inferred type is Int but Byte was expected

println(b)

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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

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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

"""

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String concatenation

val numberOfDogs = 3

val numberOfCats = 2

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=> I have 3 dogs and 2 cats

"I have $numberOfDogs dogs" + " and $numberOfCats cats"

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String templates

A template expression starts with a dollar sign ($) and can be a simple value:

val i = 10

println("i = $i")

=> i = 10

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Or an expression inside curly braces:

val s = "abc"

println("$s.length is ${s.length}")

=> abc.length is 3

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String template expressions

val numberOfShirts = 10

val numberOfPants = 5

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=> I have 15 items of clothing

"I have ${numberOfShirts + numberOfPants} items of clothing"

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Variables

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Variables

Powerful type inference

Mutable and immutable variables

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Let the compiler infer the type
You can explicitly declare the type if needed

Immutability not enforced, but recommended

Kotlin is a statically-typed language. The type is resolved at compile time and never changes.

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Specifying the variable type

Colon Notation

var width: Int = 12

var length: Double = 2.5

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Important: Once a type has been assigned by you or the compiler, you can't change the type or you get an error.

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Mutable and immutable variables

Mutable (Changeable)

Immutable (Unchangeable)

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var score = 10

val name = "Jennifer"

Although not strictly enforced, using immutable variables is recommended in most cases.

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var and val

var count = 1

count = 2

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val size = 1

size = 2

=> Error: val cannot be reassigned

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Conditionals

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Control flow

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Kotlin features several ways to implement conditional logic:

If/Else statements
When statements
For loops
While loops

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if/else statements

val numberOfCups = 30

val numberOfPlates = 50

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=> Not enough cups!

if (numberOfCups > numberOfPlates) {

println("Too many cups!")

} else {

println("Not enough cups!")

}

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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!")

}

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⇒ Large group of people!

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Ranges

Data type containing a span of comparable values (e.g., integers from 1 to 100 inclusive)

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Ranges are bounded

Objects within a range can be mutable or immutable

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Ranges in if/else statements

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=> 50

val numberOfStudents = 50

if (numberOfStudents in 1..100) {

println(numberOfStudents)

}

Note: There are no spaces around the "range to" operator (1..100)

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when statement

when (results) {

0 -> println("No results")

in 1..39 -> println("Got results!")

else -> println("That's a lot of results!")

}

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⇒ That's a lot of results!

As well as a when statement, you can also define a when expression that provides a return value.

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for loops

val pets = arrayOf("dog", "cat", "canary")

for (element in pets) {

print(element + " ")

}

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⇒ dog cat canary

You don’t need to define an iterator variable and increment it for each pass.

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for loops: elements and indexes

for ((index, element) in pets.withIndex()) {

println("Item at $index is $element\n")

}

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⇒ Item at 0 is dog

Item at 1 is cat

Item at 2 is canary

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for loops: step sizes and ranges

for (i in 1..5) print(i)

⇒ 12345

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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

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while loops

var bicycles = 0

while (bicycles < 50) {

bicycles++

}

println("$bicycles bicycles in the bicycle rack\n")

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⇒ 49 bicycles in the bicycle rack

do {

bicycles--

} while (bicycles > 50)

println("$bicycles bicycles in the bicycle rack\n")

⇒ 50 bicycles in the bicycle rack

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repeat loops

repeat(2) {

print("Hello!")

}

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⇒ Hello!Hello!

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Lists and arrays

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Lists

Lists are ordered collections of elements

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Elements can occur more than once in a list

List elements can be accessed programmatically through their indices

An example of a list is a sentence: it's a group of words, their order is important, and they can repeat.

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Immutable list using listOf()

Declare a list using listOf() and print it out.

val instruments = listOf("trumpet", "piano", "violin")

println(instruments)

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⇒ [trumpet, piano, violin]

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Mutable list using mutableListOf()

Lists can be changed using mutableListOf()

val myList = mutableListOf("trumpet", "piano", "violin")

myList.remove("violin")

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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.

⇒ kotlin.Boolean = true

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Arrays

Arrays store multiple items

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Array elements can be accessed programmatically through their indices

Array elements are mutable

Array size is fixed

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Array using arrayOf()

An array of strings can be created using arrayOf()

val pets = arrayOf("dog", "cat", "canary")

println(java.util.Arrays.toString(pets))

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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.

⇒ [dog, cat, canary]

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Arrays with mixed or single types

An array can contain different types.

val mix = arrayOf("hats", 2)

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An array can also contain just one type (integers in this case).

val numbers = intArrayOf(1, 2, 3)

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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))

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=> [4, 5, 6, 1, 2, 3]

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Null safety

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Null safety

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In Kotlin, variables cannot be null by default

Allow null-pointer exceptions using the !! operator

You can test for null using the elvis (?:) operator

You can explicitly assign a variable to null using the safe call operator

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Variables cannot be null

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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

In Kotlin, null variables are not allowed by default.

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Safe call operator

Declare an Int? as nullable

var numberOfBooks: Int? = null

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The safe call operator (?), after the type indicates that a variable can be null.

In general, do not set a variable to null as it may have unwanted consequences.

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Testing for null

Check whether the numberOfBooks variable is not null. Then decrement that variable.

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Now look at the Kotlin way of writing it, using the safe call operator.

var numberOfBooks = 6

if (numberOfBooks != null) {

numberOfBooks = numberOfBooks.dec()

}

var numberOfBooks = 6

numberOfBooks = numberOfBooks?.dec()

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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.

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throws NullPointerException if s is null

val len = s!!.length

Warning: Because !! will throw an exception, it should only be used when it would be exceptional to hold a null value.

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Elvis operator

Chain null tests with the ?: operator.

numberOfBooks = numberOfBooks?.dec() ?: 0

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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.

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Summary

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Summary

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

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In Lesson 1, you learned how to:

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Pathway

Practice what you’ve learned by�completing the pathway:

Lesson 1: Kotlin basics

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Android Development with Kotlin

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