Master Android App Development: Module 2 - Fundamentals of Java/Kotlin for Android

 

Table of Contents

1. Introduction to Java and Kotlin
   1.1 Why Java and Kotlin for Android?
   1.2 Syntax and Structure: Java vs. Kotlin

2. Variables, Data Types, and Control Flow
   2.1 Variables and Data Types
   2.2 Control Flow: Conditionals and Loops

3. Object-Oriented Programming (OOP) Concepts
   3.1 Classes and Objects
   3.2 Inheritance and Polymorphism
   3.3 Encapsulation and Abstraction

4. Functions, Lambdas, and Error Handling
   4.1 Functions and Methods
   4.2 Lambdas and Higher-Order Functions
   4.3 Exception Handling Best Practices

5. Android-Specific Libraries and APIs
   5.1 Key Android Libraries
   5.2 Working with Android APIs

6. Practical Exercise: Building a Simple Calculator App
   6.1 Project Setup in Android Studio
   6.2 Designing the UI
   6.3 Implementing Logic in Java
   6.4 Implementing Logic in Kotlin
   6.5 Testing and Debugging

7. Pros, Cons, and Alternatives
   7.1 Java vs. Kotlin: A Detailed Comparison
   7.2 Alternative Languages for Android

8. Best Practices for Android Development

9. Conclusion and Next Steps

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1. Introduction to Java and Kotlin

1.1 Why Java and Kotlin for Android?

Java and Kotlin are the backbone of Android app development. Java, a robust and widely-used language, has been the traditional choice for Android since its inception. Kotlin, introduced by JetBrains and endorsed by Google as the preferred language for Android since 2017, offers modern features and concise syntax, making it a favorite for developers aiming for efficiency.

Real-Life Example: Imagine you're building a food delivery app like Uber Eats. Java's stability ensures your app runs smoothly across millions of devices, while Kotlin's concise code lets you quickly implement features like order tracking or payment processing.

Why Learn Both?

• Java: Universally supported, vast community, extensive libraries.

• Kotlin: Null safety, modern syntax, reduces boilerplate code.

1.2 Syntax and Structure: Java vs. Kotlin

Java uses a verbose, structured approach, while Kotlin is concise and expressive. Let’s compare a simple "Hello, World!" program.

Java Example:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Kotlin Example:

fun main() {
    println("Hello, World!")
}

Key Differences:

• Java requires a class declaration; Kotlin does not.

• Kotlin’s fun keyword replaces Java’s public static void.

• Kotlin’s println is more concise than Java’s System.out.println.

---

2. Variables, Data Types, and Control Flow

2.1 Variables and Data Types

Variables store data, and data types define what kind of data. Both languages support similar primitive types (e.g., int, double, boolean) but differ in declaration.

Java Variable Declaration:

int age = 25;
String name = "Alice";
double price = 19.99;
boolean isStudent = true;

Kotlin Variable Declaration:

val age: Int = 25 // Immutable
var name: String = "Alice" // Mutable
val price: Double = 19.99
val isStudent: Boolean = true

Real-Life Example: In a fitness app, you might store a user’s age (int), name (String), weight (double), and membership status (boolean).

Kotlin’s Advantage: val (immutable) vs. var (mutable) encourages safer coding by preventing accidental changes.

2.2 Control Flow: Conditionals and Loops

Control flow directs the program’s execution. Both languages support if, switch (Java) or when (Kotlin), and loops (for, while).

Java Conditional Example:

int score = 85;
if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else {
    System.out.println("Grade: C");
}

Kotlin Conditional Example:

val score = 85
when {
    score >= 90 -> println("Grade: A")
    score >= 80 -> println("Grade: B")
    else -> println("Grade: C")
}

Real-Life Example: In an e-commerce app, use conditionals to apply discounts based on purchase amount (e.g., 10% off for orders over $100).

Loops Example (Java):

for (int i = 1; i <= 5; i++) {
    System.out.println("Item " + i);
}

Loops Example (Kotlin):

for (i in 1..5) {
    println("Item $i")
}

Kotlin’s Advantage: The .. operator and string interpolation ($i) make loops and string handling cleaner.

---

3. Object-Oriented Programming (OOP) Concepts

3.1 Classes and Objects

Classes are blueprints for objects. Objects are instances of classes.

Java Class Example:

public class Car {
    String model;
    int year;

    public Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    public void displayInfo() {
        System.out.println("Model: " + model + ", Year: " + year);
    }
}

Car myCar = new Car("Toyota", 2023);
myCar.displayInfo();

Kotlin Class Example:

class Car(val model: String, val year: Int) {
    fun displayInfo() {
        println("Model: $model, Year: $year")
    }
}

val myCar = Car("Toyota", 2023)
myCar.displayInfo()

Real-Life Example: In a ride-sharing app, a Car class could represent vehicles with properties like model and year, and methods like bookRide().

3.2 Inheritance and Polymorphism

Inheritance allows a class to inherit properties and methods from another. Polymorphism enables different classes to be treated as instances of a parent class.

Java Inheritance Example:

public class Vehicle {
    protected String brand;
    public void honk() {
        System.out.println("Beep!");
    }
}

public class Car extends Vehicle {
    public Car(String brand) {
        this.brand = brand;
    }
}

Kotlin Inheritance Example:

open class Vehicle {
    open val brand: String = ""
    fun honk() {
        println("Beep!")
    }
}

class Car(override val brand: String) : Vehicle()

Real-Life Example: In a gaming app, a Vehicle parent class could have child classes like Car and Bike, each with unique behaviors but sharing a honk() method.

3.3 Encapsulation and Abstraction

Encapsulation hides data using access modifiers (private, public). Abstraction simplifies complex systems by exposing only necessary details.

Java Encapsulation Example:

public class BankAccount {
    private double balance;

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }

    public double getBalance() {
        return balance;
    }
}

Kotlin Encapsulation Example:

class BankAccount {
    private var balance: Double = 0.0

    fun deposit(amount: Double) {
        if (amount > 0) {
            balance += amount
        }
    }

    fun getBalance(): Double = balance
}

Real-Life Example: In a banking app, encapsulation protects a user’s balance from unauthorized access, while methods like deposit() provide controlled interaction.

---

4. Functions, Lambdas, and Error Handling

4.1 Functions and Methods

Functions define reusable code blocks. In Java, they’re called methods and reside within classes. In Kotlin, they can be standalone or class-based.

Java Function Example:

public class MathUtils {
    public static int add(int a, int b) {
        return a + b;
    }
}

Kotlin Function Example:

fun add(a: Int, b: Int): Int {
    return a + b
}

Real-Life Example: In a shopping app, a function could calculate the total price of items in a cart.

4.2 Lambdas and Higher-Order Functions

Kotlin’s lambdas make code concise, especially for event handling in Android.

Kotlin Lambda Example:

val sum: (Int, Int) -> Int = { a, b -> a + b }
println(sum(5, 3)) // Output: 8

Real-Life Example: In a to-do list app, a lambda could filter tasks by priority without writing verbose loops.

4.3 Exception Handling Best Practices

Exception handling ensures apps don’t crash unexpectedly.

Java Exception Handling:

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero!");
}

Kotlin Exception Handling:

try {
    val result = 10 / 0
} catch (e: ArithmeticException) {
    println("Cannot divide by zero!")
}

Best Practices:

• Catch specific exceptions, not generic Exception.

• Provide meaningful error messages.

• Log errors for debugging.

Real-Life Example: In a weather app, handle network errors when fetching data from an API.

---

5. Android-Specific Libraries and APIs

5.1 Key Android Libraries

• Android Support Library: Provides backward compatibility.

• Jetpack: Modern tools like ViewModel, LiveData, and Room.

• Retrofit: Simplifies API calls.

• Glide: Efficient image loading.

5.2 Working with Android APIs

Android APIs enable interaction with device features (e.g., camera, GPS).

Example: Accessing Location (Kotlin):

import android.content.Context
import android.location.LocationManager

fun getLocation(context: Context): String {
    val locationManager = context.getSystemService(Context.LOCATION_SERVICE) as LocationManager
    return try {
        val location = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER)
        "Lat: ${location?.latitude}, Lon: ${location?.longitude}"
    } catch (e: SecurityException) {
        "Permission denied"
    }
}

Real-Life Example: A navigation app uses the Location API to provide real-time directions.

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6. Practical Exercise: Building a Simple Calculator App

Let’s build a calculator app to apply the concepts learned. We’ll create it in both Java and Kotlin.

6.1 Project Setup in Android Studio

1. Open Android Studio and create a new project.

2. Choose “Empty Activity” template.

3. Set language to Java or Kotlin.

4. Name the app “SimpleCalculator”.

6.2 Designing the UI

Create a layout with buttons for digits (0–9), operations (+, -, *, /), and a TextView for the result.

XML Layout (res/layout/activity_main.xml):

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <TextView
        android:id="@+id/resultText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="0"
        android:textSize="24sp"
        android:padding="10dp"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <Button
            android:id="@+id/buttonAdd"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:text="+"/>
        <Button
            android:id="@+id/buttonSubtract"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:text="-"/>
        <Button
            android:id="@+id/buttonMultiply"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:text="*"/>
        <Button
            android:id="@+id/buttonDivide"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:text="/"/>
    </LinearLayout>

    <EditText
        android:id="@+id/input1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Enter first number"
        android:inputType="numberDecimal"/>

    <EditText
        android:id="@+id/input2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Enter second number"
        android:inputType="numberDecimal"/>
</LinearLayout>

6.3 Implementing Logic in Java

Java Code (MainActivity.java):

import android.os.Bundle;
import android.view.View;
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 {
    private EditText input1, input2;
    private TextView resultText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        input1 = findViewById(R.id.input1);
        input2 = findViewById(R.id.input2);
        resultText = findViewById(R.id.resultText);

        Button addButton = findViewById(R.id.buttonAdd);
        Button subtractButton = findViewById(R.id.buttonSubtract);
        Button multiplyButton = findViewById(R.id.buttonMultiply);
        Button divideButton = findViewById(R.id.buttonDivide);

        addButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                calculate('+');
            }
        });

        subtractButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                calculate('-');
            }
        });

        multiplyButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                calculate('*');
            }
        });

        divideButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                calculate('/');
            }
        });
    }

    private void calculate(char operation) {
        try {
            double num1 = Double.parseDouble(input1.getText().toString());
            double num2 = Double.parseDouble(input2.getText().toString());
            double result = 0;

            switch (operation) {
                case '+':
                    result = num1 + num2;
                    break;
                case '-':
                    result = num1 - num2;
                    break;
                case '*':
                    result = num1 * num2;
                    break;
                case '/':
                    if (num2 == 0) {
                        throw new ArithmeticException("Division by zero");
                    }
                    result = num1 / num2;
                    break;
            }
            resultText.setText(String.valueOf(result));
        } catch (NumberFormatException e) {
            Toast.makeText(this, "Invalid input", Toast.LENGTH_SHORT).show();
        } catch (ArithmeticException e) {
            Toast.makeText(this, e.getMessage(), Toast.LENGTH_SHORT).show();
        }
    }
}

6.4 Implementing Logic in Kotlin

Kotlin Code (MainActivity.kt):

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

class MainActivity : AppCompatActivity() {
    private lateinit var input1: EditText
    private lateinit var input2: EditText
    private lateinit var resultText: TextView

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

        input1 = findViewById(R.id.input1)
        input2 = findViewById(R.id.input2)
        resultText = findViewById(R.id.resultText)

        findViewById<Button>(R.id.buttonAdd).setOnClickListener { calculate('+') }
        findViewById<Button>(R.id.buttonSubtract).setOnClickListener { calculate('-') }
        findViewById<Button>(R.id.buttonMultiply).setOnClickListener { calculate('*') }
        findViewById<Button>(R.id.buttonDivide).setOnClickListener { calculate('/') }
    }

    private fun calculate(operation: Char) {
        try {
            val num1 = input1.text.toString().toDouble()
            val num2 = input2.text.toString().toDouble()
            val result = when (operation) {
                '+' -> num1 + num2
                '-' -> num1 - num2
                '*' -> num1 * num2
                '/' -> {
                    if (num2 == 0.0) throw ArithmeticException("Division by zero")
                    num1 / num2
                }
                else -> 0.0
            }
            resultText.text = result.toString()
        } catch (e: NumberFormatException) {
            Toast.makeText(this, "Invalid input", Toast.LENGTH_SHORT).show()
        } catch (e: ArithmeticException) {
            Toast.makeText(this, e.message, Toast.LENGTH_SHORT).show()
        }
    }
}

6.5 Testing and Debugging

• Test Cases:

  • Enter valid numbers (e.g., 10, 5) and check results for all operations.

  • Test division by zero.

  • Enter non-numeric input to trigger error handling.

• Debugging Tips:

  • Use Android Studio’s debugger to set breakpoints.

  • Check Logcat for error logs.

  • Ensure permissions (if any) are declared in AndroidManifest.xml.

Real-Life Example: A calculator app could be part of a budgeting tool, helping users calculate expenses or savings.

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7. Pros, Cons, and Alternatives

7.1 Java vs. Kotlin: A Detailed Comparison

Java Pros:

• Mature, widely used, extensive documentation.

• Stable across all Android versions.

• Large community support.

Java Cons:

• Verbose syntax.

• No null safety, leading to potential crashes.

Kotlin Pros:

• Concise, modern syntax.

• Null safety reduces crashes.

• Interoperable with Java.

Kotlin Cons:

• Slightly steeper learning curve for beginners.

• Smaller community compared to Java.

7.2 Alternative Languages for Android

• Flutter (Dart): Cross-platform, fast UI development.

• React Native (JavaScript): Great for rapid prototyping.

• C++: Used for performance-critical apps via Android NDK.

Real-Life Example: A startup might choose Flutter for faster cross-platform development, but Kotlin for native Android performance.

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8. Best Practices for Android Development

• Code Organization: Use packages to separate UI, logic, and data layers.

• Null Safety (Kotlin): Always use val for immutable variables when possible.

• Error Handling: Catch specific exceptions and provide user-friendly messages.

• Performance: Avoid heavy computations on the main thread.

• Testing: Write unit tests for logic and UI tests for layouts.

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9. Conclusion and Next Steps

You’ve learned the fundamentals of Java and Kotlin, applied OOP concepts, and built a calculator app. Next, explore Android’s advanced features like Fragments, ViewModel, and Room in Module 3.

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