Java · Lesson 1 of 9

Java introduction

What Java is used for, the JDK, and the structure of a Java program.

Java is a strongly typed, object-oriented language that runs on the Java Virtual Machine (JVM). Code is compiled to bytecode, which the JVM runs on any operating system — “write once, run anywhere”. It powers large backend systems (Spring Boot), Android apps, banking software and big-data tools, and is one of the most hired-for skills in Technopark and Infopark.

Setup

Install a JDK (Java Development Kit) — version 17 or 21 (long-term support). Check with java -version. IntelliJ IDEA Community and VS Code are popular free editors.

Program structure

All code lives inside a class.
Execution starts at public static void main(String[] args).
Statements end with ;, blocks use { }, and Java is case-sensitive.

Example program

Run with JDK 17+: java Main.java

public class Main {
    public static void main(String[] args) {
        // Print a line of text
        System.out.println("Hello from Java!");
        System.out.println("2 + 3 = " + (2 + 3));
    }
}

Output

Hello from Java!
2 + 3 = 5

Save the code as Main.java and run java Main.java (JDK 11+ runs single files directly). Names and figures are fictional.

Key points

Java runs on the JVM via bytecode.
main() is the entry point.
Every statement ends with a semicolon.

Quick check

Where does a Java program start running?

The first line of the file The main method The constructor The import statements

Practice task

Write a program that prints your name, then the number of days in 3 weeks calculated with multiplication.

Interview corner

JDK vs JRE vs JVM?

The JVM runs bytecode. The JRE is the JVM plus core libraries needed to run programs. The JDK is the JRE plus development tools such as the compiler (javac).

Why is Java called platform independent?

Source compiles to bytecode, and any OS with a JVM can run that same bytecode.