Introduction to Java

Java is one of the most widely used programming languages in the world. It runs banking systems, Android apps, large websites, big-data tools and countless business applications. It's known for being reliable, fast, strongly typed and backed by an enormous ecosystem of libraries — and it keeps evolving, with a new version every six months.

A first look

void main() {
    System.out.println("Hello, Java!");
}

Hello, Java!

That's a complete Java program. Save it as Main.java, run java Main.java, and it prints a greeting. Lesson 3 explains every part — and the longer, traditional form you'll see in most existing code.

What makes Java Java

• Write once, run anywhere. Java code is compiled to bytecode, which runs on the Java Virtual Machine (JVM). The same compiled program runs on Windows, macOS and Linux.
• Statically typed. Every variable has a type that's checked before the program runs, so many mistakes are caught by the compiler instead of by your users.
• Object-oriented. Programs are built from classes and objects that bundle data with behaviour — though modern Java also embraces records, lambdas and functional-style streams.
• Automatic memory management. A garbage collector frees memory you no longer use.
• Huge standard library and ecosystem. Collections, networking, dates, concurrency and much more are built in, and tools like Maven, Gradle, Spring and JUnit cover almost every need.

How Java runs

Main.java  ──javac──►  Main.class (bytecode)  ──java (JVM)──►  running program
 source code             portable                  just-in-time compiled
                                                    to fast machine code

The javac compiler turns source code into bytecode. The JVM loads the bytecode and, as the program runs, compiles the busiest parts to native machine code. For single-file programs, java Main.java does both steps for you.

What is Java used for?

Java versions

A new Java version is released every March and September. Every two years one becomes a long-term support (LTS) release that companies standardise on: Java 17 (2021), Java 21 (2023) and Java 25 (2025). This course uses Java 25 and points out features that older versions lack, since you'll meet older code at work.

Java and JavaScript

Despite the name, Java and JavaScript are unrelated languages. JavaScript runs in web browsers and is dynamically typed; Java is compiled, statically typed, and mostly runs on servers and devices. The similar name was a 1990s marketing decision.

How this course works

Every example in this course was compiled and run with a real Java 25 JDK, and the output shown beneath it is the actual output. The Try it yourself editors let you edit code and copy it to run on your own machine — lesson 2 shows how to set that up in a few minutes.

Try it yourself

My name is Sam.
I am learning Java.