Head First Kotlin. A Brain-Friendly Guide

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• how to use this book: Intro
  • Who is this book for?
    • Who should probably back away from this book?
  • We know what youre thinking
  • We know what your brain is thinking
  • Metacognition: thinking about thinking
  • Heres what WE did:
  • Heres what YOU can do to bend your brain into submission
  • Read me
  • The technical review team
  • Acknowledgments
  • OReilly
• Table of Contents (the real thing)
• 1. getting started: A Quick Dip
  • Welcome to Kotlinville
    • Its crisp, concise and readable
    • You can use object-oriented AND functional programming
    • The compiler keeps you safe
  • You can use Kotlin nearly everywhere
    • Java Virtual Machines (JVMs)
    • Android
    • Client-side and server-side JavaScript
    • Native apps
  • What well do in this chapter
  • Install IntelliJ IDEA (Community Edition)
  • Lets build a basic application
    • 1. Create a new project
    • 2. Specify the type of project
    • 3. Configure the project
  • Youve just created your first Kotlin project
  • Add a new Kotlin file to the project
  • Anatomy of the main function
  • Add the main function to App.kt
    • Test drive
    • What the Run command does
  • What can you say in the main function?
  • Loop and loop and loop...
    • Simple boolean tests
  • A loopy example
    • Test drive
  • Conditional branching
  • Using if to return a value
  • Update the main function
    • Test drive
  • Code Magnets
  • Using the Kotlin interactive shell
  • You can add multi-line code snippets to the REPL
    • Its exercise time
  • Code Magnets Solution
  • Your Kotlin Toolbox
• 2. basic types and variables: Being a Variable
  • Your code needs variables
    • A variable is like a cup
  • What happens when you declare a variable
    • The value is transformed into an object...
    • ...and the compiler infers the variables type from that of the object
  • The variable holds a reference to the object
    • val vs. var revisited
  • Kotlins basic types
    • Integers
    • Floating points
    • Booleans
    • Characters and Strings
  • How to explicitly declare a variables type
    • Declaring the type AND assigning a value
  • Use the right value for the variables type
  • Assigning a value to another variable
  • We need to convert the value
    • An object has state and behavior
    • How to convert a numeric value to another type
  • What happens when you convert a value
  • Watch out for overspill
  • Store multiple values in an array
    • How to create an array
  • Create the Phrase-O-Matic application
  • Add the code to PhraseOMatic.kt
  • The compiler infers the arrays type from its values
    • How to explicitly define the arrays type
  • var means the variable can point to a different array
  • val means the variable points to the same array forever...
    • ...but you can still update the variables in the array
  • Code Magnets
  • Code Magnets Solution
  • Your Kotlin Toolbox
• 3. functions: Getting Out of Main
  • Lets build a game: Rock, Paper, Scissors
    • How the game will work
  • A high-level design of the game
  • Heres what were going to do
    • Get started: create the project
  • Get the game to choose an option
    • Create the Rock, Paper, Scissors array
  • How you create functions
    • You can send things to a function
  • You can send more than one thing to a function
    • Calling a two-parameter function, and sending it two arguments
    • You can pass variables to a function so long as the variable type matches the parameter type
  • You can get things back from a function
    • Functions with no return value
  • Functions with single-expression bodies
    • Create the getGameChoice function
  • Code Magnets
  • Code Magnets Solution
  • Add the getGameChoice function to Game.kt
  • Behind the scenes: what happens
  • The story continues
  • The getUserChoice function
    • Ask for the users choice
  • How for loops work
    • Looping through a range of numbers
    • Use downTo to reverse the range
    • Use step to skip numbers in the range
    • Looping through the items in an array
  • Ask the user for their choice
    • Use the readLine function to read the users input
  • We need to validate the users input
    • And and Or operators (&& and ||)
    • Not equals (!= and !)
    • Use parentheses to make your code clear
  • Add the getUserChoice function to Game.kt
    • Test drive
    • We need to print the results
  • Add the printResult function to Game.kt
    • Test drive
  • Your Kotlin Toolbox
• 4. classes and objects: A Bit of Class
  • Object types are defined using classes
    • You can define your own classes
  • How to design your own classes
  • Lets define a Dog class
  • How to create a Dog object
  • How to access properties and functions
    • What if the Dog is in a Dog array?
  • Create a Songs application
    • Test drive
  • The miracle of object creation
  • How objects are created
    • What the Dog constructor looks like
  • Behind the scenes: calling the Dog constructor
  • Code Magnets
  • Code Magnets Solution
  • Going deeper into properties
    • Behind the scenes of the Dog constructor
  • Flexible property initialization
  • How to use initializer blocks
  • You MUST initialize your properties
  • How do you validate property values?
    • The solution: custom getters and setters
  • How to write a custom getter
  • How to write a custom setter
  • The full code for the Dogs project
    • Test drive
  • Your Kotlin Toolbox
• 5. subclasses and superclasses: Using Your Inheritance
  • Inheritance helps you avoid duplicate code
    • An inheritance example
  • What were going to do
  • Design an animal class inheritance structure
  • Use inheritance to avoid duplicate code in subclasses
  • What should the subclasses override?
    • The animals have different property values...
    • ...and different function implementations
  • We can group some of the animals
  • Add Canine and Feline classes
  • Use IS-A to test your class hierarchy
    • Use HAS-A to test for other relationships
  • The IS-A test works anywhere in the inheritance tree
  • Well create some Kotlin animals
  • Declare the superclass and its properties and functions as open
  • How a subclass inherits from a superclass
  • How (and when) to override properties
  • Overriding properties lets you do more than assign default values
  • How to override functions
    • The rules for overriding functions
  • An overridden function or property stays open...
    • ...until its declared final
  • Add the Hippo class to the Animals project
  • Code Magnets
  • Code Magnets Solution
  • Add the Canine and Wolf classes
  • Which function is called?
  • Inheritance guarantees that all subclasses have the functions and properties defined in the superclass
    • Any place where you can use a superclass, you can use one of its subclasses instead
  • When you call a function on the variable, its the objects version that responds
  • You can use a supertype for a functions parameters and return type
  • The updated Animals code
    • Test drive
  • Your Kotlin Toolbox
• 6. abstract classes and interfaces: Serious Polymorphism
  • The Animal class hierarchy revisited
  • Some classes shouldnt be instantiated
    • Declare a class as abstract to stop it from being instantiated
  • Abstract or concrete?
  • An abstract class can have abstract properties and functions
    • We can mark three properties as abstract
  • The Animal class has two abstract functions
  • How to implement an abstract class
  • You MUST implement all abstract properties and functions
  • Lets update the Animals project
    • Test drive
  • Independent classes can have common behavior
  • An interface lets you define common behavior OUTSIDE a superclass hierarchy
  • Lets define the Roamable interface
    • Interface functions can be abstract or concrete
  • How to define interface properties
  • Declare that a class implements an interface...
    • ...then override its properties and functions
  • How to implement multiple interfaces
  • How do you know whether to make a class, a subclass, an abstract class, or an interface?
  • Update the Animals project
    • Test drive
  • Interfaces let you use polymorphism
    • Access uncommon behavior by checking an objects type
  • Where to use the is operator
    • As the condition for an if
    • In conditions using && and ||
    • In a while loop
  • Use when to compare a variable against a bunch of options
  • The is operator usually performs a smart cast
  • Use as to perform an explicit cast
  • Update the Animals project
    • Test drive
  • Your Kotlin Toolbox
• 7. data classes: Dealing with Data
  • == calls a function named equals
  • equals is inherited from a superclass named Any
    • The importance of being Any
  • The common behavior defined by Any
  • We might want equals to check whether two objects are equivalent
  • A data class lets you create data objects
    • How to create objects from a data class
  • Data classes override their inherited behavior
    • The equals function compares property values
    • Equal objects return the same hashCode value
    • toString returns the value of each property
  • Copy data objects using the copy function
  • Data classes define componentN functions...
    • ...that let you destructure data objects
  • Create the Recipes project
    • Test drive
  • Generated functions only use properties defined in the constructor
  • Initializing many properties can lead to cumbersome code
    • Default parameter values to the rescue!
  • How to use a constructors default values
    • 1. Passing values in order of declaration
    • 2. Using named arguments
  • Functions can use default values too
  • Overloading a function
    • Dos and donts for function overloading:
  • Lets update the Recipes project
  • The code continued...
    • Test drive
  • Your Kotlin Toolbox
• 8. nulls and exceptions: Safe and Sound
  • How do you remove object references from variables?
  • Remove an object reference using null
    • Why have nullable types?
  • You can use a nullable type everywhere you can use a non-nullable type
  • How to create an array of nullable types
  • How to access a nullable types functions and properties
  • Keep things safe with safe calls
  • You can chain safe calls together
    • What happens when a safe call chain gets evaluated
  • The story continues
  • You can use safe calls to assign values...
    • ...and assign values to safe calls
  • Use let to run code if values are not null
  • Using let with array items
    • Using let to streamline expressions
  • Instead of using an if expression...
    • ...you can use the safer Elvis operator
  • The !! operator deliberately throws a NullPointerException
  • Create the Null Values project
  • The code continued...
    • Test drive
  • An exception is thrown in exceptional circumstances
    • You can catch exceptions that are thrown
  • Catch exceptions using a try/catch
  • Use finally for the things you want to do no matter what
  • An exception is an object of type Exception
  • You can explicitly throw exceptions
  • try and throw are both expressions
    • How to use try as an expression
    • How to use throw as an expression
  • Code Magnets
  • Code Magnets Solution
  • Your Kotlin Toolbox
• 9. collections: Get Organized
  • Arrays can be useful...
  • ...but there are things an array cant handle
    • You cant change an arrays size
    • Arrays are mutable, so they can be updated
  • When in doubt, go to the Library
  • List, Set and Map
    • List - when sequence matters
    • Set - when uniqueness matters
    • Map - when finding something by key matters
  • Fantastic Lists...
    • ...and how to use them
  • Create a MutableList...
    • ..and add values to it
  • You can remove a value...
    • ...and replace one value with another
  • You can change the order and make bulk changes...
    • ...or take a copy of the entire MutableList
  • Create the Collections project
    • Test drive
  • Code Magnets
  • Code Magnets Solution
  • Lists allow duplicate values
  • How to create a Set
    • How to use a Sets values
  • How a Set checks for duplicates
  • Hash codes and equality
    • Equality using the === operator
    • Equality using the == operator
  • Rules for overriding hashCode and equals
  • How to use a MutableSet
  • You can copy a MutableSet
  • Update the Collections project
    • Test drive
  • Time for a Map
    • How to create a Map
  • How to use a Map
  • Create a MutableMap
    • Put entries in a MutableMap
  • You can remove entries from a MutableMap
  • You can copy Maps and MutableMaps
  • The full code for the Collections project
    • Test drive
  • Your Kotlin Toolbox
• 10. generics: Know Your Ins from Your Outs
  • Collections use generics
  • How a MutableList is defined
    • Understanding collection documentation (Or, whats the meaning of E?)
  • Using type parameters with MutableList
  • Things you can do with a generic class or interface
  • Heres what were going to do
  • Create the Pet class hierarchy
  • Define the Contest class
    • Declare that Contest uses a generic type
    • You can restrict T to a specific supertype
  • Add the scores property
    • Create the addScore function
  • Create the getWinners function
  • Create some Contest objects
    • The compiler can infer the generic type
  • Create the Generics project
    • Test drive
  • The Retailer hierarchy
  • Define the Retailer interface
  • We can create CatRetailer, DogRetailer and FishRetailer objects...
    • ...but what about polymorphism?
  • Use out to make a generic type covariant
    • Collections are defined using covariant types
  • Update the Generics project
    • Test drive
  • We need a Vet class
    • Assign a Vet to a Contest
  • Create Vet objects
    • Pass a Vet to the Contest constructor
  • Use in to make a generic type contravariant
    • Should a Vet<Cat> ALWAYS accept a Vet<Pet>?
  • A generic type can be locally contravariant
  • Update the Generics project
    • Test drive
  • Your Kotlin Toolbox
• 11. lambdas and higher-order functions: Treating Code Like Data
  • Introducing lambdas
    • What were going to do
  • What lambda code looks like
  • You can assign a lambda to a variable
    • Execute a lambdas code by invoking it
  • What happens when you invoke a lambda
  • Lambda expressions have a type
  • The compiler can infer lambda parameter types
    • You can replace a single parameter with it
  • Use the right lambda for the variables type
    • Use Unit to say a lambda has no return value
  • Create the Lambdas project
    • Test drive
  • You can pass a lambda to a function
    • Add a lambda parameter to a function by specifying its name and type
  • Invoke the lambda in the function body
    • Call the function by passing it parameter values
  • What happens when you call the function
  • You can move the lambda OUTSIDE the ()s...
    • ...or remove the ()s entirely
  • Update the Lambdas project
    • Test drive
  • A function can return a lambda
  • Write a function that receives AND returns lambdas
    • Define the parameters and return type
    • Define the function body
  • How to use the combine function
    • What happens when the code runs
    • You can make lambda code more readable
  • Use typealias to provide a different name for an existing type
  • Update the Lambdas project
    • Test drive
  • Code Magnets
  • Code Magnets Solution
  • Your Kotlin Toolbox
• 12. built-in higher-order functions: Power Up Your Code
  • Kotlin has a bunch of built-in higher-order functions
  • The min and max functions work with basic types
    • The minBy and maxBy functions work with ALL types
  • A closer look at minBy and maxBys lambda parameter
    • What about minBy and maxBys return type?
  • The sumBy and sumByDouble functions
    • sumBy and sumByDoubles lambda parameter
  • Create the Groceries project
    • Test drive
  • Meet the filter function
    • Theres a whole FAMILY of filter functions
  • Use map to apply a transform to your collection
    • You can chain function calls together
  • What happens when the code runs
  • The story continues...
  • forEach works like a for loop
  • forEach has no return value
    • Lambdas have access to variables
  • Update the Groceries project
    • Test drive
  • Use groupBy to split your collection into groups
  • You can use groupBy in function call chains
  • How to use the fold function
  • Behind the scenes: the fold function
  • Some more examples of fold
    • Find the product of a List<Int>
    • Concatenate together the name of each item in a List<Grocery>
    • Subtract the total price of items from an initial value
  • Update the Groceries project
    • Test drive
  • Your Kotlin Toolbox
  • Leaving town...
    • Its been great having you here in Kotlinville
• A. coroutines: Running Code in Parallel
  • Lets build a drum machine
    • 1. Create a new GRADLE project
    • 2. Enter an artifact ID
    • 3. Specify configuration details
    • 4. Specify the project name
    • Add the audio files
  • Add the code to the project
    • Test drive
    • Use coroutines to make beats play in parallel
  • 1. Add a coroutines dependency
    • 2. Launch a coroutine
    • Test drive
    • A coroutine is like a lightweight thread
  • Use runBlocking to run coroutines in the same scope
    • Test drive
  • Thread.sleep pauses the current THREAD
    • The delay function pauses the current COROUTINE
  • The full project code
    • Test drive
• B. testing: Hold Your Code to Account
  • Kotlin can use existing testing libraries
    • Add the JUnit library
  • Create a JUnit test class
  • Using KotlinTest
  • Use rows to test against sets of data
• C. leftovers: The Top Ten Things: (We Didnt Cover)
  • 1. Packages and imports
    • How to add a package
    • Package declarations
  • The fully qualified name
    • Type the fully qualified name...
    • ...or import it
  • 2. Visibility modifiers
    • Visibility modifiers and top level code
  • Visibility modifiers and classes/interfaces
  • 3. Enum classes
    • Enum constructors
  • enum properties and functions
  • 4. Sealed classes
    • Sealed classes to the rescue!
  • How to use sealed classes
  • 5. Nested and inner classes
  • An inner class can access the outer class members
  • 6. Object declarations and expressions
  • Class objects...
    • ...and companion objects
  • Object expressions
  • 7. Extensions
  • 8. Return, break and continue
    • Using labels with break and continue
  • Using labels with return
  • 9. More fun with functions
    • vararg
    • infix
    • inline
  • 10. Interoperability
    • Interoperability with Java
    • Using Kotlin with JavaScript
    • Writing native code with Kotlin
• Index

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