Java Study Material

What is Java

What is Java?

Java is a popular, general-purpose programming language created by James Gosling at Sun Microsystems in 1995. Oracle owns it today. Java is used to build backend servers, Android apps, banking systems, big data tools and much more.

Platform independent: write once, run anywhere (WORA). The same compiled code runs on Windows, Linux and macOS.
Object oriented: programs are built from classes and objects.
Strongly typed: every variable has a type that is checked at compile time.
Secure and robust: no pointer arithmetic, automatic memory management (garbage collection), strong exception handling.
Huge ecosystem: Spring Boot, Hibernate, Maven, Gradle, Android, Kafka, Hadoop and thousands of libraries.

Your First Look at Java

Example

Every Java program lives inside a class. The program starts running from the main method.

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
        System.out.println("Welcome to Careeroza.");
    }
}

Output

Hello, Java!
Welcome to Careeroza.

Where is Java Used?

Area	Examples
Backend / web servers	Spring Boot APIs at Flipkart, Paytm, Swiggy, Amazon
Banking and finance	core banking systems, payment gateways, trading platforms
Android apps	Android SDK is Java based (Kotlin also runs on the JVM)
Big data	Hadoop, Kafka, Spark (JVM based), Elasticsearch
Enterprise software	ERP, CRM, insurance and telecom systems
Desktop and tools	IntelliJ IDEA, Eclipse, Minecraft (Java Edition)

Tip: Java is one of the most asked languages in Indian campus placements and service-company interviews (TCS, Infosys, Wipro, Accenture). Learning it well opens many doors.

Java Versions

Java gets a new version every six months. Some versions are LTS (Long Term Support) and are used by most companies.

Version	Year	Highlights
Java 8 (LTS)	2014	lambdas, streams, new Date/Time API
Java 11 (LTS)	2018	`var` in lambdas, new HTTP client, String methods
Java 17 (LTS)	2021	records, sealed classes, text blocks, switch expressions
Java 21 (LTS)	2023	virtual threads, pattern matching for switch, record patterns

Note: This course uses Java 21. Everything in the basic level also works on Java 17.