How to write a app on Android: a complete guide

Mobile application development is one of the most in-demand skills in the modern IT industry. Millions of users around the world interact with Android devicesdaily, and the need for quality software is only growing. Writing a app for this platform opens up great opportunities both for creating your own startup and for building a successful career as a professional developer.

Many beginners mistakenly believe that creating an application requires deep knowledge of mathematics or highly complex equipment. In fact, the entry threshold is much lower than it seems at first glance. It is enough to have a computer, a desire to learn and an understanding of the basic principles of logic. Modern automation tools and extensive documentation allow the first working code already in the first days of training.

In this article we will analyze in detail the entire path from idea to finished product. You will learn what tools are needed to get started, how the language differs Kotlin from Java, and how to properly structure the project. We will not delve into the academic jungle, but will focus on practical steps that will help you create a working application with your own hands.

Preparing the working environment and choosing tools

Before writing lines of code, you need to prepare your development environment. The industry has standardized around one core app that provides all the necessary compilers, emulators, and code editors. This Android Studio is the official integrated development environment (IDE) from Google. It is free, cross-platform and regularly updated.

The installation process is quite simple, but requires attention to system requirements. For comfortable operation, your computer will need at least 8 GB of RAM, although the recommended amount is 16 GB to quickly build projects and run the emulator. You also need to allocate about 4 GB of free space on your hard drive for the app itself and additional SDK components.

After downloading the installer, run it and follow the instructions of the wizard. During the installation process, you will be asked to select components to download. Be sure to make sure that items Android SDK and Android Virtual Deviceare checked. Without an emulator, you will have to constantly connect a physical device for testing, which is not always convenient in the early stages.

⚠️ Attention: When you first launch Android Studio, it may require you to download a huge number of updates and libraries. Don't interrupt this process, even if it seems stuck. Downloading components SDK Platform and Build Tools may take considerable time depending on the speed of your Internet.

When the installation is complete, a welcome window will open in front of you. Here you can create a new project or open an existing one. The app's interface may seem overloaded with many panels and menus, but over time you will learn to customize the workspace to suit your needs. The main thing is not to be intimidated by the abundance of buttons at the start.

Selecting a programming language: Kotlin vs Java

One ​​of the first questions a novice developer faces is: what language should the application be written in? For a long time, the de facto standard was Java. It is a robust, object-oriented language with a long history and thousands of pre-built libraries. Most of the old textbooks and code examples on the Internet are written in it.

However, in 2017, Google officially declared Kotlin the preferred language for Android development. This is a modern language that is fully compatible with Java, but does not have many of its shortcomings. Kotlin code is more concise, safer and more readable. Statistics show that more than 60% of new professional projects are now created in this language.

Let's look at the key differences so you can make an informed choice:

• 🚀 Conciseness: Kotlin requires significantly fewer lines of code to perform the same tasks, which speeds up development and simplifies support.
• 🛡️ Security: Built-in protection against NullPointerException (null pointer error), which is the cause of most failures in Java applications.
• 🔄 Compatibility: You can use Java libraries inside a Kotlin project and vice versa, mixing code without problems.
• 🎓 Learning curve: For beginners, Kotlin is often clearer due to its more modern syntax and lack of unnecessary "verbosity."

If you are starting from scratch, we strongly recommend choosing Kotlin. By studying it, you immediately get used to modern industry standards. You should learn Java only if you have to support legacy code (old projects) or you plan to work in companies where the technology stack has not yet been updated.

Project structure and navigation in Android Studio

After creating a new project (“New Project”) and selecting the “Empty Activity” template, the IDE will generate a basic file structure. Understanding where everything is is critical to working effectively. The project window is usually displayed on the left, and the default view is Android, which groups files logically rather than physically.

The central location is occupied by the folder manifestscontaining the file AndroidManifest.xml. This is your application passport. Permissions to access the camera, the Internet or geolocation are specified here, and all screens (Activities) that make up the app are registered. Any change in access rights begins with editing this file.

Next is the folder java (or kotlin), where all the business logic is stored. Here you will create classes that describe the behavior of the screens. The main file is usually called MainActivity. It is in the method that onCreate the life of your application begins at startup. All the code responsible for responding to button clicks and processing data is written here.

The folder res (resources) contains all visual and static resources. It has a complex nested structure:

• 🎨 drawable: icons, pictures and vector graphics are stored here.
• 📝 layout: XML files that describe the appearance of screens (location of buttons, texts, input fields).
• 🎨 values: files with lines of text, colors and styles for design consistency.
• 📱 mipmap: application icons for different screen densities.

Component	File extension	Purpose
Manifest	.xml	Configuration of rights and application components
Activity	.kt /.java	Logic of specific operation screen
Layout	.xml	Visual layout of the user interface
Gradle	.gradle /.kts	Project build settings and dependencies

⚠️ Attention: Never change package names manually after creating a project, simply by renaming the folders. This may break references in code and build configuration. Use the Refactor → Rename function in Android Studio for safe renaming.

Creating an interface: working with XML and Layout Editor

The user sees not the code, but the interface. In Android, screen layout is done using XML (Extensible Markup Language). You can write markup code manually in a text editor or use the visual designer Layout Editorby dragging elements with the mouse.

Each interface element is called a View. The most common of them are: TextView for displaying text, Button for buttons, EditText for input fields and ImageView for pictures. These elements are located inside containers that define their relative positions. Previously, the standard was LinearLayout i RelativeLayout, but now Google is promoting a more flexible ConstraintLayout.

An example of the simplest markup with a button and text looks like this:

<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello, Android!" />

<Button
android:id="@+id/myButton"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Click me" />

The attribute android:id is critical. It is through this identifier that you will find the element in the code (in the Activity file) in order to programmatically change its properties or attach event handlers to it. Without an ID, it is impossible to connect the visual part with the logical part.

What is a ConstraintLayout?

This is a powerful container that allows you to position elements relative to each other or screen boundaries using “constraints”. It allows you to create complex interfaces without nesting, which improves application performance.

The visual editor allows you to see how the interface will look on different screen sizes. There is a preview panel at the top where you can select your phone model (for example Pixel 6 or Samsung Galaxy S23) and screen orientation. This helps to avoid situations when on a large tablet the button moves beyond the visible area.

Writing logic: revitalizing the interface

The interface itself is static. To make the application respond to user actions, you need to write code in the Activity class. The first step is to associate the variables in the code with elements from the XML markup. In Kotlin, it is convenient to use the syntax findViewById or library View Binding.

for this. Let's consider a simple example: let's make sure that when you click a button, the text on the screen changes. In the method onCreate we find the button and text, and then set the event listener (setOnClickListener). Inside this listener, an action is written that will be performed when clicked.

class MainActivity: AppCompatActivity {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)

val myText: TextView = findViewById(R.id.textView)
val myButton: Button = findViewById(R.id.myButton)

myButton.setOnClickListener {
myText.text ="Text successfully changed!"
}
}
}

This code demonstrates the basic principle of reactive programming: “when event X occurs, do Y.” The application logic is built from many such handlers. You can react to taps, swipes, text input, receiving data from the network or changing the state of the phone's sensors.

To debug the code and check the operation of variables, use the tool Logcat. It displays system logs and messages that you programmatically send to the console using the command Log.d. This is an invaluable tool for finding errors when an application does not behave as expected.

⚠️ Attention: The main thread operation (UI-thread) should not be blocked by long calculations or network requests. If you download a file from the Internet directly in the onClickmethod, the application will freeze and the system will display an ANR (Application Not Responding) error. For such tasks, use coroutines or separate threads.

Testing, building and publishing the application

When the functionality is ready, the testing stage begins. You can run the application on an emulator, which simulates a phone on your PC screen, or on a real device. A real device is preferable, since the emulator may not correctly reproduce the operation of the sensor, GPS or camera.

To connect a real phone, you need to turn it on Developer mode (usually 7 clicks on the build number in the “About phone” settings) and allow USB debugging. Once the cable is connected, the phone will appear in the list of devices in the top bar of Android Studio. Pressing the green “Play” button will install and launch the application.

The final stage is assembling the release version. In the menu Build select item Generate Signed Bundle / APK. You will need to create a signing key (Keystore). Keep this key in a safe place: without it, you will not be able to update your application in the future if you lose the key file.

The resulting file .aab (Android App Bundle) is uploaded to console Google Play Console. Before publishing, you need to fill out the application card: add a description, screenshots, privacy policy and age rating. The moderation process on Google Play takes from several hours to several days.

Remember that the release of the first version is not the end, but the beginning. Users will find bugs, propose new features, and Android versions will be updated. Maintaining an application requires constant work on the code and adaptation to new platform requirements.

How long does it take to write the first application?

To create a simple calculator application or task list with a basic interface, a beginner will need 1 to 3 weeks of intensive training and practice. Creating a complex product with a server part and a database can take several months.

Do you need to know English for development?

Desirable. All official documentation, best courses and most bug answers on StackOverflow are written in English. A basic technical level (reading documentation) is quite enough to get started, but knowledge of the language will speed up progress significantly.

Is it possible to develop applications on a phone?

Technically, there are environments like AIDE or Sketchware, but they are extremely limited. For serious development, you need a computer (Windows, macOS or Linux) with Android Studio installed. Mobile tools are only suitable for teaching basic syntax.

Is it free to publish applications on Google Play?

No. A one-time fee of $25 is required to register for a developer account. After payment, you get access to the console forever and can publish an unlimited number of applications without a monthly fee.