How to write code in Java for Android: a complete guide

Creating mobile applications for the most popular operating system in the world begins with choosing the right tool. Despite the active promotion of the Kotlin language by Google, Java remains the foundation of the Android ecosystem. Millions of lines of legacy code, educational materials and stability make this language an ideal choice for beginners who want to understand the architecture of mobile development from the inside.

The process of writing code requires not only knowledge of the syntax, but also an understanding of how the application interacts with the operating system. You will learn how to work with the activity lifecycle, memory management, and creating user interfaces. This goes from simply printing "Hello World" text to complex network interactions with servers.

In this guide, we'll walk you through the key development steps so you can write your first working project. We will not delve into the academic jungle of theory, but will focus on the practical aspects necessary to run an application on a real device or emulator.

Preparing the development environment and installing the SDK

The first step towards creating an application is installing specialized software. The gold standard of the industry is the environment Android Studio, built on the basis of IntelliJ IDEA. It provides all the necessary tools: code editor, visual layout editor, emulator and debugger.

At the first launch, the installation wizard will offer to download Android SDK (Software Development Kit). This is a set of libraries, tools and documentation, without which code compilation is impossible. Please note that the size of downloaded files can exceed several gigabytes, so make sure you have a stable Internet connection.

It is important to choose the right version of the platform. Although new versions of Android are released every year, learning and building most applications requires you to select the latest stable version of the API. In the project settings, you can specify minSdkVersion —the minimum version of the system on which your application will run.

⚠️ Attention: Library versions and system requirements in Android Studio are frequently updated. Before installation, check the official system requirements on the developer's website, as older computers may not be able to handle new versions of the emulator.

After installation, create a new project by selecting a template Empty Activity. This is a blank sheet on which we will write our code. The project structure may seem complicated, but the main files with which you will work are located in the folder app/src/main/java for logic and app/src/main/res/layout for the interface.

Basics of project structure and manifest files

Each Android application has a unique configuration file called AndroidManifest.xml. This file tells the system about your application components: activities, services, broadcast recipients, and content providers. Without the correct configuration of the manifest, the application simply will not start.

The manifest also defines the permissions required for the app to operate. If you plan to use internet, camera, or geolocation access, you must explicitly request these rights. Android's security system strictly controls access to sensitive user data.

Consider an example of requesting permission to access a network. This code is added inside the tag <manifest>:

<uses-permission android:name="android.permission.INTERNET" />

In addition to permissions, the main entry point to the application is registered in the manifest - MainActivity. It is the one that launches first when you click on the application icon in the launcher. An error in declaring an activity in this file will cause the application to crash immediately upon launch.

Writing application logic in Java

The application logic is written in classes that inherit from the base class Activity. This class manages the life cycle of an application window. The key method is onCreatewhich is called by the system when creating an activity. This is where the variables are initialized and the interface is bound.

To work with interface elements, the findViewByIdmethod is used. It finds the widget in the XML markup by its unique identifier id. Once found, you can programmatically change the element's properties: text, color, visibility, or add click handlers.

Here is an example of simple code that changes the text on a button when clicked:

Button myButton = findViewById(R.id.my_button);
myButton.setOnClickListener(new View.OnClickListener {
@Override
public void onClick(View v) {
myButton.setText("Clicked!");
}
});

Note the use of anonymous inner classes for handling events. This is the classic approach in Java, although more modern versions of the language can use lambda expressions to shorten the code. Understanding the work of event listeners (Listeners) is critical to creating interactive applications.

Creating a User Interface in XML

The appearance of the application is described in format markup files XML. This is a declarative language where you describe what elements should be on the screen and how they are arranged. The basic building blocks are called View (buttons, text fields) and ViewGroup (containers for other elements).

The most common container is LinearLayoutwhich arranges elements either vertically or horizontally. For more complex and adaptive interfaces, ConstraintLayoutis used, which allows you to position elements relative to each other and screen borders.

Each interface element can be set with width and height attributes. The value match_parent causes the element to stretch to the full available size of its parent, and wrap_content shrinks the element to the size of its contents. Selecting these options correctly ensures that the application will look good on different screen sizes.

Element (View)	Description	Typical usage
TextView	Text display	Titles, descriptions, tags
EditText	Text input field	Registration forms, search
Button	Action button	Confirmation, navigation
ImageView	Image display	Logos, photographs

Don’t forget about resources. All lines, colors and sizes should be stored in separate files in the folder res/values. This allows you to easily localize the application into other languages ​​and change the theme without rewriting the logic code.

⚠️ Attention: Never “sew” rigid lines of text directly into the XML interface markup. Always use string resources (@string/name), otherwise you will encounter problems when translating the application into other languages.

Building the project and running it on the emulator

When the code is written and the markup is created, the compilation stage begins. The build system takes the Java source code, resources and libraries, and turns them into an installation package Gradle takes Java source code, resources and libraries, and turns them into an installation package .apk or .aab. This process may take some time, especially during the first build when all dependencies are downloaded.

You do not need to have a physical device to test. The emulator built into Android Studio allows you to run a virtual smartphone directly on your computer. You can select your device model, Android version, and even network and GPS settings.

To launch the application, click the green "Play" button in the toolbar. If the emulator has not yet been created, the wizard will prompt you to set up a virtual device (AVD). It is recommended to select a system image without symbols Google Play for the purity of tests, if you do not need specific services.

What to do if the emulator is slow?

Enable virtualization (VT-x or AMD-V) in the BIOS of your computer. Without hardware acceleration, the emulator will work extremely slowly, making development impossible.

If the application starts but does not work correctly, use the tool Logcat. It displays system logs in real time. Errors in the code, such as NullPointerExceptionwill be displayed here in red, indicating the exact line where the failure occurred.

Debugging errors and optimizing code

Writing code without errors is an almost impossible task for a beginner. The process of finding and fixing bugs is called debugging. Android Studio allows you to install breakpoints (breakpoints). When such a line is reached, app execution pauses and you can check the values ​​of the variables.

Memory leaks are a common problem. In Java, the garbage collector automatically removes unused objects, but if you store references to activities or contexts in static variables, the memory will not be freed. This may cause the system to crash the application.

Use a profiler to analyze performance. It will show how much memory your application consumes, how much CPU usage there is, and how often the interface is redrawn. Optimizing these parameters is critical for working on weak devices.

It is also worth paying attention to the warnings of the code analyzer Lint. It highlights potentially dangerous designs, unused resources, and broken practices before the application launches. Ignoring these warnings may lead to unstable operation in the future.

Publishing the application on Google Play

The final stage is preparing the application for release. To do this, you need to build a signed version of the package. You will need to create a signing key (Keystore) that will identify you as the developer. Losing this key will make it impossible to update your application in the futureas new versions must be signed with the same certificate.

Before publishing, you must fill out the application page in the Google Play Developer Console. This includes a description, screenshots, icon and privacy policy. Moderation may take from several hours to several days.

It is worth considering that the application store rules are constantly changing. Requirements for the target API level (Target SDK) are updated annually. An application compiled for an outdated version of Android will simply not be accepted for publication.

⚠️ Attention: API level requirements (Target SDK) and privacy policies are regularly updated by Google. Before submitting for moderation, be sure to check the current terms in the developer console to avoid the build being rejected.

After successful moderation, your application will become available to millions of users. However, the developer's work does not end there: it is necessary to monitor feedback, answer questions and release updates with bug fixes.

Frequently asked questions about Java development for Android

Do I need to know Kotlin if I am learning Java?

Knowledge of Java is an excellent base, as the syntax Kotlin is similar in many ways and runs on the same virtual machine. However, for employment in modern projects, knowledge of Kotlin is becoming a mandatory requirement, since Google is positioning it as a priority language.

Is it possible to develop applications on an old computer?

Yes, it is possible, but with limitations. You will have to stop using a heavy emulator and test applications on a real device connected via USB. It is also recommended to disable unnecessary plugins in Android Studio to save RAM.

How long does it take to write your first application?

A simple application, such as a calculator or a task list, can be written in 1-2 weeks of intensive training. Creating a complex product with a database and network interaction can take several months.

Where to look for ready-made libraries for Java?

The main source of libraries is the repository Maven Central i JitPack. They are connected via file build.gradle. Popular libraries simplify working with the network (Retrofit), loading images (Glide) and databases (Room).