WIXLIB

Type erasureReified type parametersThe startActivity methodStar-projectionsType parameter naming conventionsSummary7.Extension Functions and PropertiesExtension functionsExtension functions under the hoodNo method overridingAccess to receiver elementsExtensions are resolved staticallyCompanion object extensionsOperator overloading using extension functionsWhere should top-level extension functions be used?Extension propertiesWhere should extension properties be used?Member extension functions and propertiesType of receiversMember extension functions and properties under the hoodGeneric extension functionsCollection processingKotlin collection type hierarchyThe map, filter, flatMap functionsThe forEach and onEach functionsThe withIndex and indexed variantsThe sum, count, min, max, and sorted functionsOther stream processing functionsExamples of stream collection processingSequenceFunction literals with receiverKotlin standard library functionsThe let functionUsing the apply function for initializationThe also functionThe run and with functionThe to functionDomain-specific languageAnkoSummary

8.DelegatesClass delegationDelegation patternDecorator patternProperty delegationWhat are delegated properties?Predefined delegatesThe lazy functionThe notNull functionThe observable delegateThe vetoable delegateProperty delegation to Map typeCustom delegatesView bingingPreference bindingProviding a delegateSummary9.Making Your Marvel Gallery ApplicationMarvel GalleryHow to use this chapterMake an empty projectCharacter galleryView implementationNetwork definitionBusiness logic implementationPutting it all togetherCharacter searchCharacter profile displaySummary

PrefaceNowadays, the Android application development process is quite extensive.Over the last few years, we have seen how various tools have evolved to makeour lives easier. However, one core element of Android application developmentprocess hasn’t changed much over time, Java. The Android platform adapts tonewer versions of Java, but to be able to use them, we need to wait for a verylong time until new Android devices reach proper market propagation. Also,developing applications in Java comes with its own set of challenges since Javais an old language with many design issues that can’t be simply resolved due tobackward compatibility constraints.Kotlin, on the other hand, is a new but stable language that can run on allAndroid devices and solve many issues that Java cannot. It brings lots of provenprogramming concepts to the Android development table. It is a great languagethat makes a developer's life much easier and allows to produce more secure,expressive, and concise code.This book is an easy-to-follow, practical guide that will help you to speed up andimprove the Android development process using Kotlin. We will present manyshortcuts and improvements over Java and new ways of solving commonproblems. By the end of this book, you will be familiar with Kotlin features andtools, and you will be able to develop an Android application entirely in Kotlin.

What this book coversBeginning Your Kotlin Adventure, discusses Kotlin language, its featuresand reasons to use it. We'll introduce reader to the Kotlin platform and show howKotlin fits into Android development process.Chapter 1,Laying a Foundation, is largely devoted to the building blocks of theKotlin. It presents various constructs, data types, and features that make Kotlinan enjoyable language to work with.Chapter 2,Playing with Functions, explains various ways to define and call afunction. We will also discuss function modifiers and look at possible locationswhere function can be defined.Chapter 3,Chapter 4,Classes and Objects, discusses the Kotlin features related to objectoriented programming. You will learn about different types of class. We will alsosee features that improve readability: properties operator overloading and infixcalls.Chapter 5,Functions as First-Class Citizens, covers Kotlin support for functionalprogramming and functions as first-class citizens. We will take a closer look atlambdas, higher order functions, and function types.Chapter 6,Generics Are Your Friends, explores the subjects of generic classes,interfaces, and functions. We will take a closer look at the Kotlin generic typesystem.Chapter 7,Extension Functions and Properties, demonstrates how to add newbehavior to an existing class without using inheritance. We will also discusssimpler ways to deal with collections and stream processing.Chapter 8,Delegates, shows how Kotlin simplifies class delegation due to built-inlanguage support. We will see how to use it both by using built-in propertydelegates and by defining custom ones.Chapter 9,Making Your Marvel Gallery Application, utilizes most of the features

discussed in the book and use it to build a fully functional Android application inKotlin.

What you need for this bookTo test and use the code presented in this book, you need only Android Studioinstalled. Chapter 1, Beginning your Kotlin Adventure, explains how a new projectcan be started and how the examples presented here can be checked. It alsodescribes how most of the code presented here can be tested without anyprogram installed.

Who this book is forTo use this book, you should to be familiar with two areas:You need to know Java and object-oriented programming concepts,including objects, classes, constructors, interfaces, methods, getters, setters,and generic types. So, if this area does not ring a bell, it will be difficult tofully understand the rest of this book. Start instead with an introductoryJava book and return to this book afterward.Though not mandatory, understanding the Android platform is muchdesirable because it will help you to understand the presented examples inmore detail, and you’ll have deeper understanding the problems that aresolved by Kotlin. If you are an Android developer with 6-12 months ofexperience or you have created few Android applications, you’ll be fine. Onthe other hand, if you feel comfortable with OOP concepts but yourknowledge of Android platform is limited, you will probably still be OK formost of the book.Being open-minded and eager to learn new technologies will be very helpful. Ifsomething makes you curious or catches your attention, feel free to test it andplay with it while you are reading this book

ConventionsIn this book, you will find a number of text styles that distinguish betweendifferent kinds of information. Here are some examples of these styles and anexplanation of their meaning.Code words in text, database table names, folder names, filenames, fileextensions, pathnames, dummy URLs, user input, and Twitter handles are shownas follows: "Let's look at the range data type, which allows to define endinclusive ranges."A block of code is set as follows:val capitol "England" to "London"println(capitol.first) // Prints: Englandprintln(capitol.second) // Prints: LondonWhen we wish to draw your attention to a particular part of a code block, therelevant lines or items are set in bold:ext.kotlin version '1.1.3'repositories {maven { url 'https://maven.google.com' }jcenter()}Any command-line input or output is written as follows:sdk install kotlinNew terms and important words are shown in bold. Words that you see on thescreen, for example, in menus or dialog boxes, appear in the text like this: "Setname, package, and location for the new project. Remember to tick IncludeKotlin support option."Warnings or important notes appear like this.Tips and tricks appear like this.

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Beginning Your Kotlin AdventureKotlin is great language that makes Android development easier, faster, andmuch more pleasant. In this chapter, we will discuss what Kotlin really is andlook at many Kotlin examples that will help us build even better Androidapplications. Welcome to the amazing journey of Kotlin, that will change theway you think about writing code and solving common programming problems.In this chapter, we will cover the following topics:First steps with KotlinPractical Kotlin examplesCreating new Kotlin project in Android StudioMigrating existing Java project to KotlinThe Kotlin standard library (stdlib)Why Kotlin is a good choice to learn

Say hello to KotlinKotlin is a modern, statically typed, Android-compatible language that fixesmany Java problems, such as null pointer exceptions or excessive codeverbosity. Kotlin is a language inspired by Swift, Scala, Groovy, C#, and manyother languages. Kotlin was designed by JetBrains professionals, based onanalysis of both developers experiences, best usage guidelines (most importantare clean code and effective Java), and data about this language's usage. Deepanalysis of other programming languages has been done. Kotlin tries hard to notrepeat the mistakes from other languages and take advantage of their most usefulfeatures. When working with Kotlin, we can really feel that this is a mature andwell-designed language.Kotlin takes application development to a whole new level by improving codequality and safety and boosting developer performance. Official Kotlin supportfor the Android platform was announced by Google in 2017, but the Kotlinlanguage has been here for some time. It has a very active community and Kotlinadoption on the Android platform is already growing quickly. We can describeKotlin as a safe, expressive, concise, versatile, and tool-friendly language thathas great interoperability with Java and JavaScript. Let's discuss these features:Safety: Kotlin offers safety features in terms of nullability andimmutability. Kotlin is statically typed, so the type of every expression isknown at compile time. The compiler can verify that whatever property ormethod that we are trying to access or a particular class instance actuallyexists. This should be familiar from Java which is also statically typed, butunlike Java, Kotlin type system is much more strict (safe). We have toexplicitly tell the compiler whether the given variable can store null values.This allows making the program fail at compile time instead of throwing aNullPointerException at runtime:

Easy debugging: Bugs can be detected much faster during the developmentphase instead of crashing the application after it is released and thusdamaging the user experience. Kotlin offers a convenient way to work withimmutable data. For example, it can distinguish mutable (read-write) andimmutable (read-only) collections by providing convenient interfaces(under the hood collections are still mutable).Conciseness: Most of the Java verbosity was eliminated. We need less codeto achieve common tasks and thus the amount of boilerplate code is greatlyreduced, even comparing Kotlin to Java 8. As a result, the code is alsoeasier to read and understand (expressive).Interoperability: Kotlin is designed to seamlessly work side by side withJava (cross-language project). The existing ecosystem of Java libraries andframeworks works with Kotlin without any performance penalties. ManyJava libraries have even Kotlin-specific versions that allow more idiomaticusage with Kotlin. Kotlin classes can also be directly instantiated andtransparently referenced from Java code without any special semantics andvice versa. This allows us to incorporate Kotlin into existing Androidprojects and use Kotlin easily together with Java (if we want to).Versatility: We can target many platforms, including mobile applications(Android), server-side applications (backend), desktop applications,frontend code running in the browser, and even build systems (Gradle).Any programming language is only as good as its tool support. Kotlin hasoutstanding support for modern IDEs such as Android Studio, IntelliJ Idea, andEclipse. Common tasks like code assistance or refactoring are handled properly.The Kotlin team works hard to make the Kotlin plugin better with every singlerelease. Most of the bugs are quickly fixed and many of the features requestedby the community are implemented.Kotlin bug tracker: https://youtrack.jetbrains.com/issues/KTKotlin slack channel: http://slack.kotlinlang.org/Android application development becomes much more efficient and pleasantwith Kotlin. Kotlin is compatible with JDK 6, so applications created in Kotlinrun safely even on old Android devices that precede Android 4.Kotlin aims to bring you the best of both worlds by combining concepts and

elements from both procedural and functional programming. It follows manyguidelines are described in the book, Effective Java, 2nd Edition, by JoshuaBloch which is considered must read a book for every Java developer.On top of that, Kotlin is open sourced, so we can check out the project and beactively involved in any aspect of the Kotlin project such as Kotlin plugins,compilers, documentations or Kotlin language itself.

Awesome Kotlin examplesKotlin is really easy to learn for Android developers because the syntax issimilar to Java and Kotlin often feels like natural Java evolution. At thebeginning, a developer usually writes Kotlin code having in mind habits fromJava, but after a while, it is very easy to move to more idiomatic Kotlinsolutions. Let's look at some cool Kotlin features, and see where Kotlin mayprovide benefits by solving common programming tasks in an easier, moreconcise, and more flexible way. We have tried to keep examples simple and selfexplanatory, but they utilize content from various parts of this book, so it's fine ifthey are not fully understood at this point. The goal of this section is to focus onthe possibilities and present what can be achieved by using Kotlin. This sectiondoes not necessarily need to fully describe how to achieve it. Let's start witha variable declaration:var name "Igor" // Inferred type is Stringname "Marcin"Notice that Kotlin does not require semicolons. You can still use them, but theyare optional. We also don't need to specify a variable type because it's inferredfrom the context. Each time the compiler can figure out the typefrom the context we don't have to explicitly specify it. Kotlin is a strongly typedlanguage, so each variable has an adequate type:var name "Igor"name 2 // Error, because name type is StringThe variable has an inferred String type, so assigning a different value (integer)will result in compilation error. Now, let's see how Kotlin improves the way toadd multiple strings using string templates:val name "Marcin"println("My name is name") // Prints: My name is MarcinWe need no more joining strings using the character. In Kotlin, we can easilyincorporate single variable or even whole expression into string literals:val name "Igor"println("My name is {name.toUpperCase()}")// Prints: My name is IGOR

In Java any variable can store null values. In Kotlin strict null safety forces us toexplicitly mark each variable that can store nullable values:var a: String "abc"a null // compilation errorvar b: String? "abc"b null // It is correctAdding a question mark to a data type (string versus string?), we say thatvariable can be nullable (can store null references). If we don't mark variable asnullable, we will not be able to assign a nullable reference to it. Kotlin alsoallows to deal with nullable variables in proper ways. We can use safe calloperator to safely call methods on potentially nullable variables:savedInstanceState?.doSomethingThe method doSomething will be invoke

Driver Android app. He is an organizer of Kotlin User Group Singapore who has developed apps and games for Android since 2008. He is a Kotlin and RxJava addict, and obsessed with elegant and functional style code. He is mainly focused on mobile apps architecture. Stepan is making a