Android Development with Kotlin

Kotlin is a modern, statically typed, Android-compatible language that fixes many Java problems, such as null pointer exceptions or excessive code verbosity. Kotlin is a language inspired by Swift, Scala, Groovy, C#, and many other languages. Kotlin was designed by JetBrains professionals, based on analysis of both developers experiences, best usage guidelines (most important are clean code and effective Java), and data about this language's usage. Deep analysis of other programming languages has been done. Kotlin tries hard to not repeat the mistakes from other languages and take advantage of their most useful features. When working with Kotlin, we can really feel that this is a mature and well-designed language.

Kotlin takes application development to a whole new level by improving code quality and safety and boosting developer performance. Official Kotlin support for the Android platform was announced by Google in 2017, but the Kotlin language has been here for some time. It has a very active community and Kotlin adoption on the Android platform is already growing quickly. We can describe Kotlin as a safe, expressive, concise, versatile, and tool-friendly language that has great interoperability with Java and JavaScript. Let's discuss these features:

Any programming language is only as good as its tool support. Kotlin has outstanding support for modern IDEs such as Android Studio, IntelliJ Idea, and Eclipse. Common tasks such as code assistance or refactoring are handled properly. The Kotlin team works hard to make the Kotlin plugin better with every single release. Most of the bugs are quickly fixed and many of the features requested by the community are implemented.

Android application development becomes much more efficient and pleasant with Kotlin. Kotlin is compatible with JDK 6, so applications created in Kotlin run 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 many guidelines as described in the book, Effective Java, 2nd Edition, by Joshua Bloch, which is considered a must-read book for every Java developer.

On top of that, Kotlin is open sourced, so we can check out the project and be actively involved in any aspect of the Kotlin project such as Kotlin plugins, compilers, documentation, or the Kotlin language itself.

Kotlin is really easy to learn for Android developers because the syntax is similar to Java and Kotlin often feels like a natural Java evolution. At the beginning, a developer usually writes Kotlin code by having in mind habits from Java, but after a while, it is very easy to move to more idiomatic Kotlin solutions. Let's look at some cool Kotlin features and see where Kotlin may provide benefits by solving common programming tasks in an easier, more concise, and more flexible way. We have tried to keep examples simple and self-explanatory, but they utilize content from various parts of this book, so it's fine if they are not fully understood at this point. The goal of this section is to focus on the possibilities and present what can be achieved by using Kotlin. This section does not necessarily need to fully describe how to achieve it. Let's start with a variable declaration:

    var name = "Igor" // Inferred type is String 
    name = "Marcin" 

Notice that Kotlin does not require semicolons. You can still use them, but they are optional. We also don't need to specify a variable type because it's inferred from the context. Each time, the compiler can figure out the type from the context; we don't have to explicitly specify it. Kotlin is a strongly typed language, so each variable has an adequate type:

    var name = "Igor" 
    name = 2 // Error, because name type is String 

The variable has an inferred String type, so assigning a different value (integer) will result in a compilation error. Now, let's see how Kotlin improves the way to add multiple strings using string templates:

    val name = "Marcin" 
    println("My name is $name") // Prints: My name is Marcin 

We need no more joining strings using the + character. In Kotlin, we can easily incorporate single variable or even whole expressions, 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 to explicitly mark each variables, that can store nullable values:

    var a: String = "abc"
    a = null // compilation error

    var b: String? = "abc"
    b = null // It is correct

By adding a question mark to a data type (string versus string?), we say that the variable can be nullable (can store null references). If we don't mark the variable as nullable, we will not be able to assign a nullable reference to it. Kotlin also allows us to deal with nullable variables in proper ways. We can use the safe call operator to safely call methods on potentially nullable variables:

    savedInstanceState?.doSomething 

The method doSomething will be invoked only if savedInstanceState has a non-null value, otherwise the method call will be ignored. This is Kotlin's safe way to avoid null pointer exceptions that are so common in Java.

Kotlin also has several new data types. Let's look at the Range data type that allows us to define end inclusive ranges:

    for (i in 1..10) { 
        print(i) 
    } // 12345678910 

Kotlin introduces the Pair data type that, combined with infixnotation, allows us to hold a common pair of values:

    val capitol = "England" to "London" 
    println(capitol.first) // Prints: England 
    println(capitol.second) // Prints: London 

We can deconstruct it into separate variables using destructive declarations:

    val (country, city) = capitol 
    println(country) // Prints: England 
    println(city) // Prints: London 

We can even iterate through a list of pairs:

    val capitols = listOf("England" to "London", "Poland" to "Warsaw") 
    for ((country, city) in capitols) { 
        println("Capitol of $country is $city") 
    } 
 
    // Prints: 
    // Capitol of England is London 
    // Capitol of Poland is Warsaw 

Alternatively, we can use the forEach function:

    val capitols = listOf("England" to "London", "Poland" to "Warsaw") 
    capitols.forEach { (country, city) -> 
        println("Capitol of $country is $city") 
    } 

Note that Kotlin distinguishes between mutable and immutable collections by providing a set of interfaces and helper methods (List versus MutableList, Set versus Set versus MutableSet, Map versus MutableMap, and so on):

    val list = listOf(1, 2, 3, 4, 5, 6) // Inferred type is List 
    val mutableList = mutableListOf(1, 2, 3, 4, 5, 6) 
    // Inferred type  is MutableList 

Immutable collection means that the collection state can't change after initialization (we can't add/remove items). Mutable collection (quite obviously) means that the state can change.

With lambda expressions, we can use the Android framework build in a very concise way:

    view.setOnClickListener { 
        println("Click") 
    } 

The Kotlin standard library (stdlib) contains many functions that allow us to perform operations on collections in a simple and concise way. We can easily perform stream processing on lists:

    val text = capitols.map { (country, _) -> country.toUpperCase() } 
                       .onEach { println(it) } 
                       .filter { it.startsWith("P") } 
                       .joinToString (prefix = "Countries prefix P:")
    // Prints: ENGLAND POLAND
    println(text) // Prints: Countries prefix P: POLAND
    .joinToString (prefix = "Countries prefix P:")

Notice that we don't have to pass parameters to a lambda. We can also define our own lambdas that will allow us to write code in a completely new way. This lambda will allow us to run a particular piece of code only in Android Marshmallow or newer:

    inline fun supportsMarshmallow(code: () -> Unit) { 
        if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) 
        code() 
    } 
 
    //usage 
    supportsMarshmallow { 
        println("This code will only run on Android Nougat and newer") 
    } 

We can make asynchronous requests easily and display responses on the main thread using the doAsync function:

    doAsync { 
        var result = runLongTask()  // runs on background thread 
 
        uiThread { 
            toast(result)           // run on main thread 
        } 
    } 

Smart casts allow us to write code without performing redundant casting:

    if (x is String) { 
        print(x.length) // x is automatically casted to String 
    } 
 
    x.length //error, x is not casted to a String outside if block 
 
    if (x !is String) 
        return 
 
    x.length // x is automatically casted to String 

The Kotlin compiler knows that the variable x is of the type String after performing a check, so it will automatically cast it to the String type, allowing it to call all methods and access all properties of the String class without any explicit casts.

Sometimes, we have a simple function that returns the value of a single expression. In this case, we can use a function with an expression body to shorten the syntax:

    fun sum(a: Int, b: Int) = a + b 
    println (sum(2 + 4)) // Prints: 6 

Using default argument syntax, we can define the default value for each function argument and call it in various ways:

    fun printMessage(product: String, amount: Int = 0, 
        name: String = "Anonymous") { 
        println("$name has $amount $product")  
    } 
 
    printMessage("oranges") // Prints: Anonymous has 0 oranges 
    printMessage("oranges", 10) // Prints: Anonymous has 10 oranges 
    printMessage("oranges", 10, "Johny") 
    // Prints: Johny has 10 oranges 

The only limitation is that we need to supply all arguments without default values. We can also use named argumentsyntax to specify function arguments:

    printMessage("oranges", name = "Bill") 

This also increases readability when invoking the function with multiple parameters in the function call.

The data classes give a very easy way to define and operate on classes from the data model. To define a proper data class, we will use the data modifier before the class name:

    data class Ball(var size:Int, val color:String) 
 
    val ball = Ball(12, "Red") 
    println(ball) // Prints: Ball(size=12, color=Red) 

Notice that we have a really nice, human readable string representation of the class instance and we do not need the new keyword to instantiate the class. We can also easily create a custom copy of the class:

    val ball = Ball(12, "Red") 
    println(ball) // prints: Ball(size=12, color=Red) 
    val smallBall = ball.copy(size = 3) 
    println(smallBall) // prints: Ball(size=3, color=Red) 
    smallBall.size++ 
    println(smallBall) // prints: Ball(size=4, color=Red) 
    println(ball) // prints: Ball(size=12, color=Red) 

The preceding constructs make working with immutable objects very easy and convenient.

One of the best features in Kotlin are extensions. They allow us to add new behavior (a method or property) to an existing class without changing its implementation. Sometimes when you work with a library or framework, you would like to have an extra method or property for a certain class. Extensions are a great way to add those missing members. Extensions reduce code verbosity and remove the need to use utility functions known from Java (for example, the StringUtils class). We can easily define extensions for custom classes, third-party libraries, or even Android framework classes. First of all, ImageView does not have the ability to load images from a network, so we can add the loadImage extension method to load images using the Picasso library (an image loading library for Android):

    fun ImageView.loadUrl(url: String) { 
        Picasso.with(context).load(url).into(this) 
    } 
 
    //usage 
    imageView.loadUrl("www.test.com\\image1.png") 

We can also add a simple method displaying toasts to the Activity class:

    fun Context.toast(text:String) { 
        Toast.makeText(this, text, Toast.LENGTH_SHORT).show() 
    } 
 
    //usage (inside Activity class)
    toast("Hello") 

There are many places where usage of extensions will make our code simpler and more concise. Using Kotlin, we can fully take advantage of lambdas to simplify Kotlin code even more.

Interfaces in Kotlin can have default implementations as long as they don't hold any state:

    interface BasicData { 
        val email:String 
        val name:String 
        get() = email.substringBefore("@") 
    } 

In Android, there are many applications where we want to delay object initialization until it is needed (used). To solve this problem, we can use delegates:

    val retrofit by lazy { 
        Retrofit.Builder() 
            .baseUrl("https://www.github.com") 
            .addConverterFactory(MoshiConverterFactory.create()) 
            .build() 
    } 

Retrofit (a popular Android networking framework) property initialization will be delayed until the value is accessed for the first time. Lazy initialization may result in faster Android application startup times, since loading is deferred to when the variable is accessed. This is a great way to initialize multiple objects inside a class, especially when not all of them are always needed (for certain class usage scenarios, we may need only specific objects) or when not every one of them is needed instantly after class creation.

All the presented examples are only a glimpse of what can be accomplished with Kotlin. We will learn how to utilize the power of Kotlin throughout this book.

There are multiple ways of managing and running Kotlin code. We will mainly focus on Android Studio and Kotlin Playground.

Kotlin Playground

The fastest way to try Kotlin code without the need to install any software is Kotlin Playground (https://try.kotlinlang.org). We can run Kotlin code there using JavaScript or JVM Kotlin implementations and easily switch between different Kotlin versions. All the code examples from the book that does not require Android framework dependencies and can be executed using Kotlin Playground.

The main function is the entry point of every Kotlin application. This function is called when any application starts, so we must place code from the book examples in the body of this method. We can place code directly or just place a call to another function containing more Kotlin code:

    fun main(args: Array<String>) { 
        println("Hello, world!") 
    }

Note

Android applications have multiple entry points. The main function is called implicitly by the Android framework, so we can't use it to run Kotlin code on the Android platform.

Android Studio

All Android Studio's existing tools work with Kotlin code. We can easily use debugging, lint checks, have proper code assistance, refactoring, and more. Most of the things work the same way as for Java, so the biggest noticeable change is the Kotlin language syntax. All we need to do is to configure Kotlin in the project.

Android applications have multiple entry points (different intents can start different components in the application) and require Android framework dependencies. To run the book examples, we need to extend the Activity class and place code there.

Configuring Kotlin for the project

Starting from Android Studio 3.0, full tooling support for Kotlin was added. Installation of the Kotlin plugin is not required and Kotlin is integrated even deeper into the Android development process.

To use Kotlin with Android Studio 2.x, we must manually install the Kotlin plugin. To install it, we need to go to Android Studio | File |Settings | Plugins | Install JetBrains plugin... | Kotlin and press the Install button:

To be able to use Kotlin, we need to configure Kotlin in our project. For existing Java projects, we need to run the Configure Kotlin in project action (the shortcut in Windows is Ctrl+Shift+A, and in macOS, it is command + shift + A) or use the corresponding Tools | Kotlin|Configure Kotlin in Project menu item:

Then, select Android with Gradle:

Finally, we need to select the required modules and the proper Kotlin version:

The preceding configuration scenario also applies to all existing Android projects that were initially created in Java. Starting from Android Studio 3.0, we can also check the Include Kotlin support checkbox while creating a new project:

In both scenarios, the Configure Kotlin in project command updates the root build.gradle file and the build.gradle files corresponding to the module(s) by adding Kotlin dependencies. It also adds the Kotlin plugin to the Android module. At the time of writing this book release version of Android Studio 3 is not yet available, but we can review the build script from the pre-release version:

//build.gradle file in project root folder 
buildscript { 
    ext.kotlin_version = '1.1' 
 
    repositories { 
        google() 
        jcenter() 
    } 
    dependencies { 
       classpath 'com.android.tools.build:gradle:3.0.0-alpha9' 
       classpath "org.jetbrains.kotlin:kotlin-gradle-
             plugin:$kotlin_version" 
    } 
} 
  
...  
//build.gradle file in the selected modules 
apply plugin: 'com.android.application' 
apply plugin: 'kotlin-android' 
apply plugin: 'kotlin-android-extensions'
... 
dependencies { 
    ...
    implementation 'com.android.support.constraint:constraint-
          layout:1.0.2'
         
} 
... 

Note

Prior to the Android plugin for Gradle 3.x (delivered with Android Studio 3.0), compile dependency configuration was used instead of implementation.

To update the Kotlin version (let us say in the future), we need to change the value of the kotlin_version variable in the build.gradle file (project root folder). Changes in Gradle files mean that the project must be synchronized, so Gradle can update its configuration and download all the required dependencies:

Using Kotlin in a new Android project

For new Kotlin projects created in Android Studio 3.x, the main activity will be already defined in Kotlin, so that we can start writing Kotlin code right away:

Adding a new Kotlin file is similar to adding a Java file. Simply right-click on a package and select new | Kotlin File/Class:

Note

The reason why the IDE says Kotlin File/Class and not simply Kotlin class, which is analogous to Java class, is that we can have more members defined inside a single file. We will discuss this in more detail in Chapter 2, Laying a Foundation.

Notice that Kotlin source files can be located inside the java source folder. We can create a new source folder for Kotlin, but it is not required:

Running and debugging a project is exactly the same as in Java and does not require any additional steps besides configuring Kotlin in the project:

Starting from Android Studio 3.0, various Android templates will also allow us to select a language. This is the new Configure Activity wizard:

Java to Kotlin converter (J2K)

Migration of existing Java projects is also quite easy, because we can use Java and Kotlin side by side in the same project. There are also ways to convert existing Java code into Kotlin code by using the Java to Kotlin converter (J2K).

The first way is to convert whole Java files into Kotlin files using the convert Java File to Kotlin command (the keyboard shortcut in Windows is Alt + Shift + Ctrl + K and in macOS is option + shift + command + K), and this works very well. The second way is to paste Java code into an existing Kotlin file and the code will also be converted (a dialog window will appear with a conversion proposition). This may be very helpful when learning Kotlin.

If we don't know how to write a particular piece of code in Kotlin, we can write it in Java, then simply copy to the clipboard and paste it into the Kotlin file. Converted code will not be the most idiomatic version of Kotlin, but it will work. The IDE will display various intentions to convert the code even more and improve its quality. Before conversion, we need to make sure that the Java code is valid, because conversion tools are very sensitive and the process will fail even if a single semicolon is missing. The J2K converter combined with Java interoperability allows Kotlin be introduced gradually into the existing project (for example, to convert a single class at a time).

Alternative ways to run Kotlin code

Android Studio offers an alternative way of running Kotlin code without the need to run the Android application. This is useful when you want to quickly test some Kotlin code separately from the long Android compilation and deployment process.

The way to run Kotlin code is to use the Kotlin Read Eval Print Loop (REPL). REPL is a simple language shell that reads single user input, evaluates it, and prints the result:

REPL looks like the command-line, but it will provide us with all the required code hints and will give us access to various structures defined inside the project (classes, interfaces, top-level functions, and so on):

The biggest advantage of REPL is its speed. We can test Kotlin code really quickly.

We will focus mainly on Android, but keep in mind that Kotlin can be compiled to multiple platforms. Kotlin code can be compiled to Java bytecode and then to Dalvik bytecode. Here is the simplified version of the Kotlin build process for the Android platform:

For pure Kotlin projects, only the Kotlin compiler will be used, but Kotlin also supports cross-language projects, where we can use Kotlin together with Java in the same Android project. In such cases, both compilers are used to compile the Android application and the result will be merged at the class level.

Kotlin has strong commercial support from JetBrains, a company that delivers very popular IDEs for many popular programming languages (Android Studio is based on JetBrains IntelliJ IDEA). JetBrains wanted to improve the quality of their code and team performance, so they needed a language that would solve all the Java issues and provide seamless interoperability with Java. None of the other JVM languages meet those requirements, so JetBrains finally decided to create their own language and started the Kotlin project. Nowadays, Kotlin is used in their flagship products. Some use Kotlin together with Java while others are pure Kotlin products.

Kotlin is quite a mature language. In fact, its development started many years before Google announced official Android support (the first commit dates back to 2010-11-08):

After the version 1.0 release in 2016, more and more companies started to support the Kotlin project. Gradle added support for Kotlin into building scripts; Square, the biggest creator of Android libraries posted that they strongly support Kotlin; and finally, Google announced official Kotlin support for the Android platform. This means that every tool that will be released by the Android team will be compatible not only with Java but also with Kotlin. Google and JetBrains have begun a partnership to create a nonprofit foundation for Kotlin, responsible for future language maintenance and development. All of this will greatly increase the number of companies that use Kotlin in their projects.

Kotlin is also similar to Apple's Swift programming language. In fact, such is the resemblance that some articles focus on differences, not similarities. Learning Kotlin will be very helpful for developers eager to develop applications for Android and iOS. There are also plans to port Kotlin to iOS (Kotlin/Native), so maybe we don't have to learn Swift after all. Full stack development is also possible in Kotlin, so we can develop server-side applications and frontend clients that share the same data model as mobile clients.

We've discussed how the Kotlin language fits into Android development and how we can incorporate Kotlin into new and existing projects. We have seen useful examples where Kotlin simplified the code and made it much safer. There are still many interesting things to discover.

In the next chapter, we will learn about Kotlin building blocks and lay a foundation to develop Android applications using Kotlin.