Five words people use interchangeably, and should not

Term	What it actually is
Java language	The syntax and rules, defined by the Java Language Specification.
Java platform	The language plus the standard library plus the runtime.
JDK	Java Development Kit — the compiler `javac`, tools, and a JRE. What developers install.
JRE	Java Runtime Environment — enough to run Java, not to compile it.
JVM	The virtual machine that executes bytecode. The engine itself.

Write once, run anywhere

javac turns your .java source into a .class file of bytecode — instructions for an imaginary machine. The JVM for each real platform reads that same bytecode and executes it. Your compiled file is portable; the JVM is what differs.

Compiled and interpreted, both

Bytecode starts out interpreted. The JVM watches which methods run often and its JIT compiler turns those into native machine code. Long-running Java programs get faster as they run, which is why benchmarks must warm up before measuring.

Java SE, Jakarta EE, OpenJDK, Oracle JDK

Java SE is the standard edition — the language and core libraries. Jakarta EE (formerly Java EE) adds enterprise specifications on top. OpenJDK is the open-source reference implementation; Oracle JDK is Oracle's build of essentially the same code under a different licence. Most teams run an OpenJDK build.

LTS releases

A new Java version ships every six months, but only some are Long-Term Support: 8, 11, 17, 21 and 25. Production systems overwhelmingly run LTS versions. This course targets Java 21, an LTS release, and flags where older versions differ.