Android Development in Kotlin

Kotlin Introduction

Kotlin is an open-source programming language for Android, JVM, JavaScript, and Native. The software project began in 2010 and has always been open source. The Apache 2.0 license governs its development.

Both object-oriented and functional constructs are available in Kotlin. The user may use it in both OO and FP forms or a mixture of the two. If the user wants to explore functional programming, Kotlin is a great option. It has first-class support for features like higher-order functions, function types, and lambdas.

Kotlin over Java

Kotlin is a more formal language. The number of lines of code can be reduced by around 40%, according to rough estimates. It’s also more type-safe; for example, support for non-nullable types reduces the likelihood of NPEs in applications.

Brilliant casting, higher-order functions, extension functions, and lambdas with receivers are just a few of the features that make it easier to write expressive code and construct DSL using Kotlin.

However, Kotlin is fully interoperable with the Java programming language, and developers take reasonable steps to ensure that the current codebase communicates appropriately with Kotlin. Kotlin code converts easily from Java code and vice versa. The IDE also provides an automatic Java-to-Kotlin converter, which simplifies the migration of existing code.

Moreover, learning Kotlin is relatively easier for beginners because of various factors such as its type inference, extensive standard library, syntax, and explicit nullability. Moreover, as codebases, libraries, and tech giants such as Google move more towards Kotlin, it is becoming a highly desirable and growing language for android development.

Kotlin For Android Development

Kotlin is a programming language that is in use for server-side, client-side web, and Android development. It supports other platforms as well, such as embedded systems, macOS, and iOS, which is on the way, thanks to Kotlin/Native. Kotlin is used by developers for mobile development primarily. It also supports client-side applications with JavaScript and data science.

On Android, Kotlin is available as a first-class language. Hundreds of Android apps, including Basecamp, Pinterest, and others, also use Kotlin. Kotlin is fully supported by Android Studio, allowing users to connect Kotlin files to your current project and convert Java code to Kotlin. Users can then use all Android Studio’s existing software, such as autocomplete, lint scanning, refactoring, debugging, and more, with the Kotlin code.

Kotlin in Android Studio

New Project

Android Studio makes it simple to build Android apps for various devices, including phones, tablets, TVs, and wearables.

If the user already has a project open, the user can start working on a new one by going to File > New > New Project from the main menu. The Build New Project wizard appears next, allowing the user to pick the type of project they want to start and populating it with code and resources. This page walks through using the Build New Project wizard to create a new project.

As shown below, the next move is to configure some settings and construct a new project.

Choose a name for the new project.
Give the package a name. This package name is also the application ID by default, which the user can modify later.
Choose the Save location to store the project locally.
In Android Studio, choose the language to use when writing sample code for the new project. Bear in mind that the project does not have to build using just that language.
Choose the lowest API level that the software can allow. The Application encompasses less modern Android APIs if the user chooses a lower API level. However, the program may run on a more significant percentage of Android devices.

To get started, Android Studio creates a new project with some basic code and resources. If the user later decides to support a particular interface form factor, the user may add a module to the project.

Add Kotlin with an Existing Project

Kotlin is fully supported by Android Studio, allowing users to connect Kotlin files to existing projects and convert Java code to Kotlin. After that, you can use all of Android Studio’s software with the Kotlin file.

Choose one of the numerous Android models, such as a new blank Fragment, by going to File > New.

Select Kotlin as the Source Language in the wizard that appears. If the user wants to create a new task, the New Android Activity dialogue appears.

Alternatively, the user can build a simple Kotlin file by going to File > New > Kotlin File/Class. Select the java directory in the Project browser. The New Kotlin File/Class window allows the user to specify the file name and various file form options, including File, Class, Interface, and Object.

Upon adding the Kotlin class for the first time, Android studio prompts an error message that ‘Kotlin is not configured in the project.’

Configure Kotlin by pressing the Configure button in the editor’s upper right corner of the lower-right corner of the event log warning.

When prompted, select the option to configure Kotlin for all modules that contain Kotlin files.

Kotlin adds to the project’s classpath, and the Kotlin Android plugin applies to each module, which contains Kotlin files.

// Project build.gradle file.
buildscript {
    ext.kotlin_version = '1.4.10'
    ...
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

// Inside each module using kotlin
plugins {
    ...
    id 'kotlin-android'
}
...
dependencies {
   implementation 'androidx.core:core-ktx:1.3.2'
   implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}

Java to Kotlin Code

Open the Java file in Android Studio and choose code> Convert Java File to Kotlin File to convert it to Kotlin. Build a new Kotlin file (File > New > Kotlin File/Class) and paste the Java code into that File instead. Android Studio prompts and offers to convert the code to Kotlin. To convert, choose Yes.

Nullability and Code Conversion

View objects and other components are often delayed in Android initialization until the fragment or operation to which they are attached reaches the required lifecycle state. However, the button variable is nullable. However, In the example below, it should never be null.

The Kotlin code treats Button as a nullable form because its value may not allocate at construction time. For this scenario, this conversion is less desirable than latent since it requires the user to unwrap the button reference with a non-null statement or safe-call operator any time it is accessed.

public class JavaFragment extends Fragment {
    // Null until onCreateView.
    private Button button;
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_content, container,false);
        // Get a reference to the button in the view, only after the root view is inflated.
        button = root.findViewById(R.id.button);
        return root;
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        // Not null at this point of time when onViewCreated runs
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ...
            }
        });
    }

Basic Android Application Using Kotlin

Following are the steps to develop a basic android application using Kotlin in the android studio:

Start a new android project by selecting this option in the welcome dialog box or choosing it from the file option in the top menu.
Select the basic activity from the template and select the dialog box for new project creation.
In the next dialog box, write the application name of the user’s choice, such as myKotlinApplication.
Select Kotlin as the project’s language. This action will add the necessary Kotlin files to the new project for development. Leave other options as default and finish the steps.
Android Studio will create a project folder with all the required folders and files for the application development. Moreover, it will set up the screen layout based on the template choice. The users can change the layout by editing the “.xml” file included in the project folder.
The users can build on top of the basic template by editing the activity files in the com.example.projectname files. The users can add the back-end functionality of the existing activities and their components inside these files, and create more activities and connect them using Kotlin language. The users can leave the files as they are to run the basic android application.
The next step is to create an emulator or virtual device to run the application. The users can connect their Android phones to the computer to run the application, a faster way to test the applications. However, the emulator provides the option to test the application on various android versions and mobile phones of the users’ choice.
Click the AVD Manager in the tools menu and create a new emulator, or use an already created emulator.
Select the run option in the run app or click the play button in the toolbar to run the application.
Choose the select device in the run option to switch between various devices. Some devices may require the user to permit to install the corresponding platform.
The android studio will build the application and then run it with the layout chosen to create the application /project.
The users can explore the layout folder to build the application’s user interface, the values folder for the resources such as colors, menus to create various menus, and navigation for the navigation graph.