Java

In simple terms

Java is built around two big ideas: nearly everything is an object, and your code runs on a virtual machine instead of directly on the hardware. You compile Java once into bytecode, and that same bytecode runs unchanged anywhere a Java Virtual Machine (JVM) exists — Windows, macOS, Linux, a phone, a mainframe. That “write once, run anywhere” promise, plus industrial-strength tooling and decades of libraries, made Java the backbone of enterprise software and the original language of Android.

More detail

Java was created at Sun Microsystems by James Gosling and released in 1995. Its defining properties:

• Statically typed and class-based object-oriented — types are checked at compile time, and code is organized into classes.
• Compiled to JVM bytecode by javac, not to native machine code. The JVM interprets that bytecode and JIT-compiles hot paths to native code at runtime, so long-running Java approaches C++ speed.
• Garbage collected — a generational collector (G1 by default; ZGC and Shenandoah for very low pause times) frees memory automatically.
• Verbose but predictable, with an enormous ecosystem: Spring, Hibernate, and the Maven/Gradle build tools.

The real star is the JVM — one of the most heavily optimized managed runtimes ever built. It’s a shared platform: Kotlin, Scala, Clojure, and Groovy all compile to the same bytecode and run on it. Modern Java (records, sealed classes, and virtual threads in Java 21+) is far more concise than the ceremony-heavy Java of 2005, while keeping near-total backward compatibility.

Why it matters

Java runs an enormous share of the world’s backend systems — banks, trading platforms, telecom, government, and big-tech services — and was the native language of Android for over a decade. Learning it also means learning the JVM, a mature, observable, ferociously optimized runtime that an entire family of languages depends on.

Real-world examples

• Much of Netflix’s backend runs as JVM microservices.
• Core data infrastructure — Apache Kafka, Elasticsearch, Hadoop, Cassandra — is written in Java.
• Android apps were historically written in Java (now mostly Kotlin, also on the JVM).
• Minecraft: Java Edition is, famously, Java.

Common misconceptions

• “Java and JavaScript are related.” Only by a 1990s marketing decision — they’re completely different languages with different designers, runtimes, and uses.
• “Java is slow.” Startup is heavyweight and memory use is high, but the JIT makes steady-state server Java competitive with C++ on many workloads.

Learn next

Java’s compile-time checking is a strong type system; its automatic memory model is garbage collection; and javac plus the JVM is a compiler toolchain in action.