Introduction to Programming in Java

Java is one of the most popular and widely-used programming languages in the world. Known for its platform independence, robustness, and object-oriented design, Java is used for building web applications, mobile apps (Android), enterprise systems, and more. Its “write once, run anywhere” philosophy makes it a versatile choice for developers.

This guide will introduce you to the fundamentals of Java programming, helping you write clean, efficient, and maintainable code.

Table of Contents

1. What is Java?
2. Setting Up Your Environment
3. Variables and Data Types
4. Control Flow
5. Functions (Methods)
6. Arrays and Collections
7. Object-Oriented Programming
8. Exception Handling
9. File I/O
10. Multithreading

What is Java?

Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is designed to be platform-independent, meaning Java programs can run on any device with a Java Virtual Machine (JVM).

// Example: Your first Java program
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, Java Programming!");
    }
}

Setting Up Your Environment

To write and run Java programs, you need the Java Development Kit (JDK) installed on your system. You can download it from the official Oracle website.

# Example: Compiling and running a Java program
javac Main.java
java Main

Variables and Data Types

Java is statically typed, meaning variable types are determined at compile time. Common data types include int, double, String, and boolean.

// Example: Declaring variables
public class Main {
    public static void main(String[] args) {
        int age = 25;
        double height = 5.9;
        String name = "Alice";
        boolean isStudent = true;

        System.out.println("Name: " + name + ", Age: " + age + ", Height: " + height);
    }
}

Control Flow

Java supports if, else, switch, and loops (for, while, do-while) for controlling program flow.

// Example: Conditional statement
public class Main {
    public static void main(String[] args) {
        int age = 18;

        if (age >= 18) {
            System.out.println("You are an adult.");
        } else {
            System.out.println("You are a minor.");
        }
    }
}

Functions (Methods)

Functions in Java are called methods and are defined within classes. Methods can return values or perform actions.

// Example: Method
public class Main {
    static int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        int result = add(5, 10);
        System.out.println("Result: " + result); // 15
    }
}

Arrays and Collections

Java provides arrays and collections like ArrayList, HashMap, and HashSet for storing and manipulating data.

// Example: ArrayList
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);

        for (int number : numbers) {
            System.out.println("Number: " + number);
        }
    }
}

Object-Oriented Programming

Java is an object-oriented language, supporting classes, inheritance, polymorphism, and encapsulation.

// Example: Class and Object
class Person {
    String name;
    int age;

    void display() {
        System.out.println("Name: " + name + ", Age: " + age);
    }
}

public class Main {
    public static void main(String[] args) {
        Person person = new Person();
        person.name = "Alice";
        person.age = 25;
        person.display();
    }
}

Exception Handling

Java uses try, catch, and finally blocks to handle exceptions gracefully.

// Example: Exception handling
public class Main {
    public static void main(String[] args) {
        try {
            int result = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Error: " + e.getMessage());
        } finally {
            System.out.println("Execution complete.");
        }
    }
}

File I/O

Java provides classes like FileReader, FileWriter, and BufferedReader for reading from and writing to files.

// Example: Reading from a file
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        try {
            File file = new File("example.txt");
            Scanner scanner = new Scanner(file);
            while (scanner.hasNextLine()) {
                System.out.println(scanner.nextLine());
            }
            scanner.close();
        } catch (FileNotFoundException e) {
            System.out.println("File not found.");
        }
    }
}

Multithreading

Java supports multithreading, allowing you to run multiple threads concurrently for better performance.

// Example: Thread
class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running.");
    }
}

public class Main {
    public static void main(String[] args) {
        MyThread thread = new MyThread();
        thread.start();
    }
}