Mobile application development remains one of the most popular areas in the IT industry, and the Android platform occupies a leading position here. Despite the growing popularity of the Kotlin language, Java still is the foundation of the operating system and a huge number of existing applications. If you decide to start your journey in mobile development, understanding the principles of Java in the Android environment is a mandatory basis, which will give you access to millions of lines of code and proven architectural solutions.
You don’t need heavy-duty hardware to get started, but having a modern computer with enough RAM is critical. The process of compiling and emulating devices is quite resource-intensive. In this article, we will go through all the steps: from installing the development environment to creating the first working application that can be run on a real smartphone or virtual device.
Preparing the development environment and installing tools
The first step towards creating applications is installing an integrated development environment (IDE). The unconditional de facto standard in the world of Android development is Android Studio. This environment was created by Google specifically for working with the Android platform and contains all the necessary tools: a code editor, a visual interface designer, an emulator and a debugger.
The distribution should be downloaded exclusively from the developer’s official website to avoid security problems and lack of important updates. After downloading the installer, the installation process proceeds as standard, but you should pay attention to the choice of components. The IDE comes with Android SDK (Software Development Kit), which contains the libraries necessary for compiling code for different versions of the operating system.
Pay special attention to setting up the emulator. A virtual device allows you to test an application without connecting a physical phone. However, if your computer does not support hardware virtualization or has low memory, the emulator may be slow. In this case, it is better to immediately prepare a real device by enabling developer mode on it.
To speed up the emulator, make sure that virtualization technology (Intel VT-x or AMD-V) is enabled in your computer's BIOS. Without this option, the emulator will work in software mode, which is extremely slow.
After installation, you need to configure environment variables, although modern versions of Android Studio often do this automatically. Check that the path to the Java Development Kit (JDK) is specified correctly, since it is responsible for compiling your code into bytecode that can be understood by the Android virtual machine.
Creating the first project and file structure
Once you launch Android Studio, select the option to create a new project. The creation wizard will prompt you to select a template. For beginners, the ideal template is Empty Activity, which creates the minimum necessary structure without unnecessary code. You will need to specify the application name, package (domain name in reverse order, for example, com.example.myapp) and programming language - select Java.
The project structure may seem complicated due to the abundance of folders, but there is a clear logic in it. The main Java source codes are stored in the javadirectory, and all visual elements and resources are stored in the resfolder. The file AndroidManifest.xml is the passport of your application: it describes access rights, components and compatibility settings.
Why do we need the AndroidManifest file?
This file tells the Android system about the components of your application (Activity, Service, BroadcastReceiver) and requests necessary permissions, such as access to the Internet or camera. Without correctly setting up the manifest, the application simply will not start or will crash when trying to perform a protected action.
The entry point of any application is a class that inherits from Activity. In the method onCreate the interface is initialized and the start logic occurs. This is where you connect the app code with the visual part created in XML.
It is important to understand the division of responsibility: the operating logic is written in Java, and the appearance is described in XML files. This allows designers and developers to work in parallel, and also makes it easier to support different screens and themes.
User interface (UI) basics
The interface in Android is based on hierarchy of View objects. The root element is most often ConstraintLayout or LinearLayout. They contain buttons, text fields, images and other controls. Each element has a unique identifier (android:id), which is necessary to access it from Java code.
The method findViewByIdis used to connect the code and the interface. It finds an element by its ID and returns a reference to an object of the corresponding type. For example, to access a button, you need to cast the result to type Button. After this, you can hang up event listeners such as click or long press.
⚠️ Attention: Never access UI elements from a background thread. All operations with the UI must be performed strictly in the main thread (Main Thread), otherwise the application will crash with an error
CalledFromWrongThreadException.
The modern approach to layout involves using ViewBinding or DataBinding. These technologies avoid the use of magic ID strings and make the code more secure and readable. Instead of searching for a view manually, you get an automatically generated class containing links to all layout elements.
When developing for different screens, it is necessary to use adaptive sizes. In Android, for this purpose, there are units of measurement dp (density-independent pixels) for sizes and sp (scale-independent pixels) for fonts. The use of pixels will directly lead to the fact that on high-density screens the elements will become microscopic.
☑️ Check the interface before launching
Activity lifecycle and event handling
Understanding the lifecycle is the key to creating stable applications. Activity goes through several states: creation, starting, stopping, resuming and destroying. The Android system can destroy an activity at any time, for example, when there is a lack of memory or the screen is rotated, so it is important to save the data correctly.
The main lifecycle methods that you will override most often:
- 🔄
onCreate()— called when the activity is first created, initialization occurs here; - ▶️
onStart()— the activity becomes visible to the user; - ⏸️
onPause()—the activity loses focus, but is still visible (for example, a pop-up window has appeared); - ⏹️
onStop()—the activity is completely hidden; - 💀
onDestroy()—the activity is destroyed by the system or the user.
If the user rotates the device, the system by default destroys the current activity and creates a new one with a different orientation. To save entered data or game state, you must use the onSaveInstanceStatemethod. An object is placed in it Bundlewhich is then passed back to onCreate or onRestoreInstanceState.
Always save critical data in the onSaveInstanceState method, as the system may kill the application process in the background without warning, especially on devices with low RAM.
Handling key presses implemented through callback interfaces. The easiest way is to implement an interface View.OnClickListener. You can do this as an anonymous inner class right inside the button setting method or implement the interface in the activity class itself.
Working with data and multithreading
Any modern application works with data: downloads it from the network, reads it from a database, or saves user settings. Performing such operations on the main thread will block the interface and the application will become unresponsive. To solve this problem, Java uses multithreading.
A classic, although slightly outdated, approach is to use a class AsyncTask. However, in modern development it is recommended to use the library Coroutines (for Kotlin) or ExecutorService i Handler for Java. The library RxJava for reactive programming is also popular.
The mechanism SharedPreferencesis used to store simple settings (authorization tokens, selected language, themes). It is a key-value store that stores data in an XML file within the application's memory. It is accessed through the activity or application context.
What is Context in Android?
Context is an abstraction for accessing global information about the application environment. Through the context, you can access resources, databases, preferences, and launch other components. Incorrect use of context (for example, storing a reference to an Activity in a static variable) leads to memory leaks.
If you need to store complex structured data, such as a contact list or chat messages, you should use a database. The standard solution is Room —a SQLite wrapper library that simplifies working with SQL queries and provides compile-time type checking.
⚠️ Attention: When working with the network, be sure to check for an Internet connection before executing the query. Attempting to execute a network request without network access will cause an exception, which can crash the application if not handled correctly.
Debugging, testing, and publishing the application
Writing code is only half the battle. The second, no less important part is finding and correcting errors. The Logcat tool in Android Studio allows you to view system logs in real time. You can filter messages by tags or severity level (Error, Warning, Info) to quickly find the cause of the failure.
Unit tests (JUnit) and instrumental tests (Espresso) are used to automate checks. Unit tests test the logic of individual methods without running the application, while instrumental tests simulate user actions on the screen. Covering the code with tests increases the reliability of the product and simplifies refactoring in the future.
The table below shows the main debugging tools and their purpose:
| Tool | Purpose | Difficulty of learning |
|---|---|---|
| Logcat | Viewing system and application logs | Low |
| Debugger | Step-by-step code execution and variable analysis | Medium |
| Profiler | Analysis of memory, CPU and network usage | High |
| Layout Inspector | Real-time UI hierarchy analysis | Medium |
When the application is ready for release, it is necessary to collect a signed release APK or AAB file. To do this, Android Studio uses the function Generate Signed Bundle / APK. You will need to create a signing key (Keystore), which must be stored in a safe place. Losing the key will make it impossible to update the application on Google Play in the future.
Never commit the keystore file and its passwords to the version control system (Git). Use environment variables or exception files (.gitignore) to prevent sensitive data from being publicly available.
Before publishing, be sure to test the application on real devices running different versions of Android. Emulators cannot always reproduce the specific behavior of hardware or the features of firmware from specific manufacturers.
Frequent errors and ways to solve them
Beginners often encounter errors NullPointerException. It occurs when you try to call a method on an object that is null. In Android, this often happens if you forgot to initialize a variable via findViewById or if the element is not found in the layout. Always check links before using.
Another common problem is Memory Leak. It occurs when a long-lived object (such as a static variable or background thread) stores a reference to a short-lived object (Activity), preventing the garbage collector from freeing the memory. Using weak links (WeakReference) and libraries like LeakCanary helps diagnose such problems.
⚠️ Warning: Since Android 9 (API level 28), the use of unencrypted HTTP traffic is prohibited by default. If your server does not support HTTPS, you will have to explicitly allow this in your network configuration, but this is considered poor security practice.
Compilation errors related to SDK versions are resolved by checking the file build.gradle. Make sure that compileSdkVersion and targetSdkVersion are up-to-date and that the library versions are compatible with each other. Dependency version conflicts are a common cause of project build failures.
Do I need to know Kotlin if I'm learning Java for Android?
Knowledge of Java is a great base, but Google has officially announced Kotlin as the language of choice for Android development starting in 2019. Many new libraries and code examples are written directly in Kotlin. However, knowing Java, you will be able to read legacy code and understand the principles of the platform, and the transition to Kotlin will take a little time, since the languages are interoperable (compatible).
How long does it take to create your first application?
With basic knowledge of Java programming, creating a simple application (for example, a calculator or a to-do list) can take from several days to a week of intensive work. Learning the basics of the platform and tools will take about 2-3 weeks of regular training.
Is it possible to develop for Android on a tablet or phone?
Technically, there are IDEs for mobile devices (for example, AIDE), but they are very limited in functionality. Full development using emulators, profilers and complex build systems requires a desktop environment (Windows, macOS or Linux).
What if the emulator is very slow?
Try creating a virtual device with an x86 or x86_64 system image instead of ARM, since emulating the phone's processor architecture on a computer is very loads the CPU. Also make sure that the emulator has enough RAM allocated and the use of the host GPU is enabled in the settings.