How to make an application for Android in Java: from scratch to APK

Mobile application development is an exciting process that opens the door to the world of high technology. If you are wondering how to make an Android application in Java, then you are on the verge of creating something really useful. Java remains one of the most popular programming languages ​​for the Android platform, providing stability and wide compatibility with millions of devices around the world.

Getting started may seem daunting due to the abundance of tools and terms, but in reality it is much simpler. You don't need to be a math genius or have a specialized education to write your first code. You just need to strictly follow the instructions, have a computer at hand and the desire to learn new things. In this article, we will analyze the entire process of creating an application from scratch, from installing the development environment to launching the finished product.

Modern tools automate many routine tasks, allowing you to focus on the logic of the app. You'll be surprised how quickly plain text code turns into a working interface on your smartphone screen. Let's get started preparing the workplace and taking the first step into the world of mobile development.

Preparing the environment and installing Android Studio

The first and most important step is installing an integrated development environment (IDE). For working with Java and Android, the de facto standard is Android Studio. This is a powerful tool from Google that contains everything you need: a code editor, a device emulator, a debugger and a project builder. The app should be downloaded exclusively from the official website of the developers to avoid malicious modifications.

The installation process takes some time, since the environment includes many components. After launching the installer, you will be asked to select components for installation. Make sure that the items related to Android Virtual Device (emulator) and latest versions SDKare checked. Without these components, launching and testing applications will be impossible.

⚠️ Attention: Android Studio is demanding on computer resources. For comfortable work, it is recommended to have at least 8 GB of RAM and free space on the SSD disk. On older HDD drives, building a project can take an unreasonably long time.

After installation is complete, you need to configure environment variables, although modern versions often do this automatically. Check that JDK (Java Development Kit) is correctly defined in the settings. If you plan to use a physical device for testing instead of an emulator, make sure that the USB debugging option is enabled in the developer menu on your smartphone.

Creating a new project and file structure

When the environment is ready, you can create the first project. In the Android Studio start window, select the New Projectoption. The creation wizard will open in front of you, where you will need to select a template. For beginners, the best option would be Empty Activityas it provides the minimum required set of files without unnecessary code.

At the next stage you will be asked to name the project and specify the package. The project name can be anything, but the package identifier (Package Name) must be unique and follow the reverse domain notation, for example, com.example.myfirstapp. The programming language is also selected here - make sure it is selected Java, not Kotlin, if your goal is to learn Java.

After creating the project, you will see a complex folder structure. The following table describing the key directories will help you understand it:

Folder/File	Purpose
manifests	Contains AndroidManifest.xml - application passport with rights and settings.
java	All source code in Java is stored here (logic work).
res	Resources: images, interface layouts (layout), strings.
Gradle Scripts	Build configuration files, dependencies and SDK versions.

It is important to understand the difference between code and resources. All visuals are stored separately from the logic, which makes it easy to change the design without rewriting the software. The file AndroidManifest.xml is the central element that informs the system about the presence of activities and the necessary permissions.

Basics of interface markup in XML

Before writing code in Java, you need to create what the user will see on the screen. The interface in Android is described in markup language XML. Open the file activity_main.xml in the folder res/layout. You can switch to Codemode to see pure XML, or use the visual editor Design.

The main markup elements are View (views) and ViewGroup (view groups). For example, a button is Button, text is TextView, and the container for them is LinearLayout or ConstraintLayout. Each element has unique attributes such as width, height, color and text.

Consider a simple example of creating a button. In XML this will look like a tag with attributes. Pay attention to the attribute android:id —it is critically important, since through this identifier we will access the element from the Java code.

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

Use ConstraintLayout allows you to create complex interfaces by fixing elements relative to each other or the edges of the screen. This makes the layout adaptive for different screen diagonals. Do not forget to set indents (margin) and sizes (layout_width) so that elements do not stick together.

⚠️ Attention: Identifiers (id) in XML and variable names in Java code should not match, but are logically related. An error in writing the ID (for example, @+id/btn instead of @+id/myButton) will result in the code not being able to find the element on the screen.

Java application logic: MainActivity class

Now let's move on to the "brain" of the application. Open the file MainActivity.java in the folder java. This class inherits from AppCompatActivity and manages the life cycle of the screen. The main method that interests us is onCreate. It starts automatically when the user opens the application.

Inside the method onCreate initialization occurs. First of all, the method setContentView(R.layout.activity_main)is called, which “pulls” our XML markup onto the screen. Without this line, the user would simply see a white screen, since Java does not know what design to display.

Next, you need to associate the Java objects with the XML elements. For this, the method findViewByIdis used. Syntactically, this looks like a type cast. For example, to work with a button, you need to declare a variable and assign the found type to it:

Button myButton = findViewById(R.id.myButton);

After this, you can attach event handlers. Most popular — OnClickListener. It responds to user clicks. Inside this handler, you write the actions that should happen: a calculation, moving to another screen, or changing text.

What is R.id.?

R is an automatically generated class that contains references to all the resources in your project. id is a subclass that stores the identifiers of interface elements. The compiler itself creates this class based on files in the res folder.

Working with Gradle and external libraries

Modern development is unthinkable without the use of third-party libraries that speed up the creation of functions. Dependency management in Android Studio is done through the build system Gradle. The project configuration is in the file build.gradle (app module level), not the project.

To add a library, you need to open this file and find the block dependencies. Lines of the form implementation 'com.example:library:version'are added there. After adding a new line, you need to click the button Sync Nowfor the environment to download the necessary files.

Using libraries allows you to avoid writing complex code from scratch. For example, to work with images, the library is often used Glide or Picasso, and for network requests - Retrofit. This significantly reduces development time and reduces the number of errors.

It is important to keep track of library versions. Using versions that are too old can lead to conflicts, and versions that are too new can lead to instability if they are in alpha testing. Always check the compatibility of the versions compileSdkVersion and libraries used.

Running, debugging and getting APK

When the code is written, it's time to check. Connect your Android smartphone via USB or run the emulator. In the top panel of Android Studio, click the green button Run (triangle). The system will assemble the project, install the application on the device and launch it.

If the application does not work correctly, use the tool Logcat. It displays system logs in real time. Errors (Exceptions) are highlighted in red. Logcat analysis is the main way to find bugs. You will see which line of code failed.

To distribute the application among friends or test it on other devices, you need to create an installation file. To do this, select Build → Build Bundle(s) / APK(s) → Build APK(s)from the menu. After the process is completed, the system will offer to show the file in the folder.

The resulting file app-debug.apk can be transferred via instant messengers or the cloud. If installing on someone else's phone, you may need permission to install from unknown sources. This is standard Android protection that needs to be confirmed.

⚠️ Attention: The file debug.apk is signed with a debug key and is not suitable for publishing on the Google Play Store. To release, you must create a signed release key (Release Build) and store it in a secure place, since losing the key means you will not be able to update the application in the future.

Frequently asked questions (FAQ)

Do you need to pay for Android Studio and Java?

No, Android Studio is distributed free of charge under the Apache 2.0 license. The Java programming language (OpenJDK) is also free and does not require the purchase of licenses for commercial use.

Is it possible to create an application without an emulator, only on a real phone?

Yes, this is even preferable. The emulator consumes a lot of PC resources. To work with a real device, you need to enable "Developer Mode" and "USB Debugging" in the Android settings, then connect the phone with a cable.

How long will it take to learn Java to create simple applications?

A basic understanding of Java syntax takes 2-3 weeks of intensive training. Creating the first simple application (calculator, task list) is possible after 1-2 months of practice after installing the environment.

What is the difference between Java and Kotlin for Android?

Kotlin is a more modern language, officially supported by Google. It is more concise and safer than Java. However, Java has not disappeared anywhere; a lot of legacy code is written in it, and knowledge of Java remains fundamental to understanding the operation of the platform.