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What Is Java?

Scope: what the language is, what the JVM does, and why Java still runs so much of the world's software. No code in this chapter — just the mental model everything else hangs on.

The one-sentence version

Java is a general-purpose programming language where you write source code once, compile it to an intermediate form called bytecode, and run that bytecode on the Java Virtual Machine (JVM) — a program that pretends to be a computer, so your code runs on Windows, macOS, and Linux without recompiling.

The three pieces

Piece What it is Analogy
JDK (Java Development Kit) The toolbox: compiler (javac), runner (java), and libraries A kitchen with oven, knives, and pantry
JVM (Java Virtual Machine) The program that runs your compiled code The oven — same dish, any kitchen that has one
JRE (Java Runtime Environment) JVM plus the standard libraries, no compiler A rented kitchen: you can cook, not renovate

You install the JDK to write Java. The JDK includes everything else. If someone says "install Java," they mean the JDK.

Why bytecode?

Most languages pick one of two strategies:

  • Compile to machine code (C, Go, Rust): fast, but you build a separate binary for every operating system and CPU.
  • Interpret the source (Python, Ruby): portable, but the interpreter re-reads your source every run.

Java splits the difference. javac compiles your .java files once, into .class files holding bytecode — compact instructions for a virtual machine. Any real machine with a JVM can run those .class files. "Write once, run anywhere" was the 1990s slogan; the honest version is "compile once, run anywhere there's a JVM" — and there's a JVM just about everywhere.

Why Java is still everywhere

  • Enterprise software. Banks, airlines, and retailers run Java services written decades ago. That code isn't going anywhere, and neither is the language.
  • Android. Android apps were Java (now Kotlin, which runs on the JVM too) for over a decade.
  • Big data. Hadoop, Spark, Kafka, and Elasticsearch are Java/JVM systems.
  • The ecosystem. Thirty years of libraries mean someone has already solved your problem — databases, web servers, PDF generation, you name it.

The honest trade-offs

  • Verbose. Java makes you spell things out. A "hello world" needs a class, a method, and a specific incantation (next chapter). Modern Java (17+) is less wordy than the Java of 2005, but it'll never win a brevity contest.
  • Memory-hungry. The JVM itself needs a few hundred megabytes before your program does anything. Fine on a server; noticeable on a tiny microcontroller.
  • Slow to start. The JVM warms up — it watches your code run, then compiles the hot parts to fast machine code. Long services love this; one-shot scripts feel the startup cost.
  • The Oracle question. Java the language is open (OpenJDK). Oracle's branded JDK has licensing terms that changed over the years. Most developers sidestep this entirely by using free OpenJDK builds like Temurin or their Linux distro's package — see Installing Java.

Which Java?

Versions come in two flavors:

  • LTS (Long-Term Support) every two years: 17, 21, 25. These get security patches for years. Boring is good — pick an LTS.
  • Feature releases every six months in between: 22, 23, 24, 26. New toys, short support life. Fine for experimenting, not for anything you must maintain.

As of late 2026, Java 25 is the current LTS (supported to 2030) and Java 21 is the previous LTS (supported to 2028). Any LTS from 17 on covers everything in this guide. If a tutorial demands a specific version, 21 is the safe conservative pick; 25 is the current one.

What this guide doesn't cover

Build tools (Maven, Gradle), web frameworks (Spring), Android development, and the module system — real topics, each big enough for its own guide. This one gets you writing Java: the language, the object model, and the standard library pieces you reach for daily.