Java Introduction

What Is Java?

Java is a high-level, object-oriented programming language designed to be platform-independent. You write source once; it compiles to bytecode that the Java Virtual Machine (JVM) runs on Windows, Linux, macOS, and more.

Pipeline:
  source (.java) → javac → bytecode (.class) → JVM → run

Slogan:   Write Once, Run Anywhere
Entry:    public static void main(String[] args)
Build:    javac HelloWorld.java
Run:      java HelloWorld

Java powers enterprise backends, Android apps, and cloud services. This page is the gateway to the Java tutorials—strings, patterns, and interview programs have their own pages.

Why Use Java?

James Gosling led Java’s design at Sun Microsystems in the 1990s (first called Oak). It shipped in 1995 with the goal of portable software across devices and operating systems.

Key milestones

How a Java Program Runs

Java separates your source from the operating system. Keep this pipeline in mind.

Cheat sheet — building blocks

What Does a Java Program Look Like?

A minimal console program lives in a class whose name matches the filename:

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

OpenJDK is the free open-source reference implementation. Install a JDK so you have both javac and java.

Java vs Python

If you know Python, map the ideas like this:

Same idea, two styles

Print a greeting:

print("Hello, World!")

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

When to choose which

Compile and Run Your First Program

Install a JDK, save the hello program as HelloWorld.java, then build and run it from a terminal in that folder.

javac HelloWorld.java
java HelloWorld

Hello, World!

How It Works

1. Compile. javac reads HelloWorld.java and writes HelloWorld.class bytecode.

2. Run. java HelloWorld starts the JVM with that class (do not type .class).

3. Entry point. The JVM calls main, which prints via System.out.println.

You need a full JDK to compile. Next, practice text handling with Java strings.

Where Is Java Used?

Common workloads where the JVM and Java ecosystem fit well:

Key Takeaways