Java Introduction

Java is one of the most used programming languages in the world. Banking systems, Android apps, Flipkart and Amazon back-ends, Infosys and TCS projects, Minecraft: all built with Java. If you want a job as a back-end or full-stack developer in India, Java is the language most companies hire for.

Why learn Java?#

• Write once, run anywhere. The same Java program runs on Windows, Mac, Linux and servers without changes.
• Huge job market. Service companies (TCS, Infosys, Wipro, Accenture) and product companies (Amazon, Flipkart, Paytm) use Java.
• Strong and safe. Java checks your types before the program runs, so many mistakes are caught early.
• Massive ecosystem. Spring Boot, Hibernate, Android, Kafka, Elasticsearch: all Java.
• Great for learning OOP. Java is built around classes and objects, the way large software is designed.

The magic trick: bytecode and the JVM#

Most languages turn your code into machine code for one type of computer. Java does something smarter:

  Hello.java   ──javac──▶   Hello.class   ──java (JVM)──▶   runs on any OS
 (your code)   compiler     (bytecode)       virtual machine

1. You write code in a .java file.
2. The compiler javac turns it into bytecode (a .class file). Bytecode is not for any specific CPU.
3. The JVM (Java Virtual Machine) reads the bytecode and runs it on your computer. There is a JVM for every operating system.

Think of bytecode as a movie with subtitles in a universal language, and the JVM as the local translator in every country.

JDK, JRE and JVM#

Name	What it is	Who needs it
JVM	Runs bytecode, manages memory, does garbage collection	Built into the JRE
JRE	JVM + standard libraries: everything needed to *run* Java	Users running Java apps
JDK	JRE + compiler (javac) + tools: everything needed to *write* Java	You, the developer

As a developer you install the JDK. Use an LTS (long-term support) version: Java 21 or 17.

Your first Java program#

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

Output:

Hello, VitalStack!

Line by line:

• public class Hello – every Java program lives inside a class. The file must be named Hello.java (same as the public class).
• public static void main(String[] args) – the entry point. The JVM starts running your program here. You will understand each word soon: – public: the JVM can call it from outside. – static: no object needed to call it. – void: it returns nothing. – String[] args: command-line arguments.
• System.out.println(...) – prints a line of text to the console.
• Every statement ends with a semicolon ; and blocks are wrapped in curly braces { }.

Printing more#

public class Intro {
    public static void main(String[] args) {
        System.out.println("Line one");
        System.out.print("Same ");
        System.out.print("line");
        System.out.println();              // just a new line
        System.out.println(10 + 20);       // 30
        System.out.println("10 + 20 = " + (10 + 20));
    }
}

println adds a new line at the end; print does not. The + joins text with values.

Comments#

// A single-line comment

/* A comment that
   spans many lines */

/**
 * A Javadoc comment: used to generate documentation.
 */

Comments are ignored by the compiler. Use them to explain why, not what.

Where Java is used#

• Back-end web development: Spring Boot REST APIs (you will learn this in this site’s Spring Boot course).
• Android apps: Android was built on Java (Kotlin now shares the stage).
• Enterprise software: banking, insurance, telecom, government systems.
• Big data: Hadoop, Kafka, Spark, Elasticsearch.
• Business apps: Jmix lets you build full apps in Java quickly.

Common mistakes#

• File name and class name don’t match (hello.java with class Hello).
• Missing semicolon or closing brace.
• Writing system.out.println instead of System.out.println.
• Forgetting the exact main signature, so the JVM can’t find where to start.

Exercise#

1. Write a program that prints your name, your city and your favourite language on three lines.
2. Print 5 * 7 = 35 using a calculation, not by typing 35.
3. Use print twice and println once to make one line: Java is fun!.

Hint for 2

System.out.println("5 * 7 = " + (5 * 7)); The brackets make Java multiply before joining text.

Interview corner#

• Why is Java platform independent? Because the compiler produces bytecode, and each OS has its own JVM that runs that bytecode.
• JDK vs JRE vs JVM? JDK = tools to develop; JRE = what you need to run; JVM = the engine inside that executes bytecode.
• Is the JVM platform independent? No! Bytecode is platform independent. The JVM itself is different for each OS.

Summary#

• Java code (.java) is compiled by javac into bytecode (.class), which runs on the JVM on any OS.
• Install the JDK (Java 21 LTS) to write Java.
• Every program needs a class and a main method; statements end with ;.

Next, you will install Java and run your first program on your own computer.