Java for Android and next steps: Android Studio, activities, layouts, events, Java vs Kotlin, Spring Boot and your learning path

With Java fundamentals and OOP, you can move into the areas where Java is used professionally: Android apps, back-end web services with Spring Boot, and enterprise systems. This unit shows how Java fits into Android development (and how it relates to Kotlin, Google's preferred Android language), walks through the structure of a simple Android app, introduces Spring Boot for building APIs, and lays out a realistic learning path and portfolio projects to get hired.

Android development overview

Java or Kotlin?

They interoperate: Java and Kotlin classes can live in the same project. Learning Java first makes Kotlin easy (see the Kotlin Android subject). For new Android apps, most teams choose Kotlin; for maintaining existing apps and back-end jobs, Java is valuable.

Android project structure

app/
├── src/main/
│   ├── AndroidManifest.xml          app name, permissions, activities
│   ├── java/ke/co/example/fees/     Java (or Kotlin) source code
│   │   └── MainActivity.java
│   └── res/
│       ├── layout/activity_main.xml  screen layouts
│       ├── values/strings.xml        text (supports translations)
│       ├── values/colors.xml, themes.xml
│       └── drawable/, mipmap/        images and app icons
└── build.gradle(.kts)               dependencies and settings

• An Activity is a screen with its lifecycle (onCreate, onStart, onResume, onPause, onStop, onDestroy).
• Resources keep text, colours and images out of code (e.g. Kiswahili translations in values-sw/strings.xml).
• The manifest declares activities and permissions (e.g. INTERNET).

A simple screen: fees calculator

res/layout/activity_main.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:padding="24dp"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <EditText android:id="@+id/inputFee" android:hint="Term fee (KSh)"
        android:inputType="numberDecimal"
        android:layout_width="match_parent" android:layout_height="wrap_content"/>

    <EditText android:id="@+id/inputPaid" android:hint="Amount paid (KSh)"
        android:inputType="numberDecimal"
        android:layout_width="match_parent" android:layout_height="wrap_content"/>

    <Button android:id="@+id/btnCalc" android:text="Calculate balance"
        android:layout_width="match_parent" android:layout_height="wrap_content"/>

    <TextView android:id="@+id/txtResult" android:textSize="20sp"
        android:layout_marginTop="16dp"
        android:layout_width="match_parent" android:layout_height="wrap_content"/>
</LinearLayout>

MainActivity.java:

package ke.co.example.fees;

import android.os.Bundle;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        EditText inputFee = findViewById(R.id.inputFee);
        EditText inputPaid = findViewById(R.id.inputPaid);
        TextView result = findViewById(R.id.txtResult);
        Button btn = findViewById(R.id.btnCalc);

        btn.setOnClickListener(v -> {
            try {
                double fee = Double.parseDouble(inputFee.getText().toString());
                double paid = Double.parseDouble(inputPaid.getText().toString());
                double balance = fee - paid;
                result.setText(balance <= 0 ? "Fees cleared ✔" : String.format("Balance: KSh %,.2f", balance));
            } catch (NumberFormatException e) {
                Toast.makeText(this, "Please enter valid amounts", Toast.LENGTH_SHORT).show();
            }
        });
    }
}

What's happening: the layout defines widgets; findViewById connects them to Java; a click listener (a lambda) reads input, calculates, validates and updates the screen.

The same calculation logic can be tested as plain Java:

class Main {
    static String feeStatus(double fee, double paid) {
        double balance = fee - paid;
        return balance <= 0 ? "Fees cleared" : String.format("Balance: KSh %,.2f", balance);
    }
    public static void main(String[] args) {
        System.out.println(feeStatus(25000, 25000));
        System.out.println(feeStatus(25000, 18750.5));
    }
}

Keeping business logic in plain Java classes makes it easy to test and reuse.

Lists and multiple screens

• RecyclerView efficiently displays long lists (students, products, transactions) using an Adapter that binds data to row layouts.
• Intents open other screens and pass data:

Intent intent = new Intent(this, StudentDetailActivity.class);
intent.putExtra("admissionNo", "ADM001");
startActivity(intent);

• Intents also open other apps: dialer (tel:), WhatsApp links, maps, the camera.

Data and networking

Never put secret API keys (e.g. M-Pesa consumer secrets) inside the app: anyone can extract them from the APK. The app should call your server, which talks to Daraja securely (see the M-Pesa lesson).

Publishing (overview)

1. Create a signed release build (Android App Bundle, .aab) in Android Studio.
2. Register a Google Play developer account.
3. Prepare the listing: name, description, screenshots, icon, privacy policy URL, content rating, data safety form.
4. Test with internal/closed testing tracks (new personal developer accounts may need a period of testing with testers before production).
5. Release to production and monitor crashes and reviews.

The Flutter, Kotlin and app development subjects cover publishing in more depth.

Java on the back end: Spring Boot

Spring Boot is the most popular Java framework for web APIs and enterprise apps. A minimal REST endpoint:

@RestController
@RequestMapping("/api/students")
public class StudentController {
    private final StudentRepository repo;
    public StudentController(StudentRepository repo) { this.repo = repo; }

    @GetMapping("/{admissionNo}")
    public ResponseEntity<Student> get(@PathVariable String admissionNo) {
        return repo.findById(admissionNo)
                   .map(ResponseEntity::ok)
                   .orElse(ResponseEntity.notFound().build());
    }
}

Generate projects at start.spring.io (choose Spring Web, Spring Data JPA, a database driver), and learn: dependency injection, REST controllers, JPA/Hibernate with MySQL/PostgreSQL, validation, Spring Security, testing with JUnit. Banks, telcos and large companies in Kenya and abroad hire Spring developers.

Professional tools

Learning path

Portfolio project ideas

• School fees tracker app: students, payments, balances (Room database), SMS reminders via your API.
• Chama/SACCO contributions API in Spring Boot with authentication and reports.
• Inventory app for a small shop with barcode scanning (camera) and low-stock alerts.
• Bus booking demo: routes, seats, booking, M-Pesa payment via your server (sandbox).

Publish code on GitHub with READMEs and screenshots (see the Git subject).

Summary

• Android apps are built in Android Studio with Kotlin (preferred for new apps) or Java (huge existing codebases); they interoperate.
• Projects have a manifest, Activities (screens), XML layouts/resources and Gradle builds; Java code connects widgets and handles events.
• Use RecyclerView for lists, Intents for screens, Room/SharedPreferences for data, Retrofit for APIs, and keep secrets on your server.
• Publish signed app bundles on Google Play after testing; Spring Boot is the leading Java back-end framework.
• Follow a path: core Java → practice → Android (with Kotlin) or Spring back end → portfolio projects on GitHub.