what java actually is

the picture

"install java". "java is slow". "we're still on java 17". three sentences, three different things called java. kinda confusing, so let's pull them apart.

think of sheet music. you compose a song (java source). a transcriber writes it down as sheet music (javac → bytecode). any musician who can read sheet music can play it, on whatever instrument they have (a JVM, on whatever OS). the sheet music never changes between performances. the musician does.

java

the language: what class, int and for mean. written down in the java language specification (the JLS).

JVM

java virtual machine. an actual program (mostly C++) that runs bytecode. it has its own spec, the JVMS, and it has never seen a line of java syntax.

JDK

java development kit. the JVM, the standard library and the tools (javac, jshell, jar...). this is what you install.

JRE

java runtime environment: the JDK without the dev tools. oracle stopped shipping a separate one with java 11 (some vendors still build them). today you install a JDK, or ship a trimmed runtime made with jlink.

OpenJDK

the open-source project where the JDK is built. temurin, corretto, oracle's JDK: all builds of the same code. which one to pick is in getting a JDK.

write once, run anywhere

here's the trick. javac does not compile your code to x86 or arm machine code. it compiles to bytecode, the instruction set of a CPU that doesn't exist. then every OS gets its own JVM, which knows how to run that made-up CPU on the real one underneath.

you can see the bytecode's fingerprint yourself. every class file on earth starts with the same four bytes:

$ javac Main.java
$ xxd Main.class | head -1
00000000: cafe babe 0000 0045 001d 0a00 0200 0307  .......E........

CAFEBABE is the magic number (an old joke from the people who designed the format). 0x45 is 69, the class file version for java 25. we'll take one apart in compile and run.

compiled or interpreted? both

people argue about this one. the honest answer is that java gets compiled twice.

that second compile is why java programs speed up after a few seconds of running (people call it warm-up), and why "java is slow" is mostly a story about startup. once the JIT has had a look, hot code runs as native machine code, sometimes better tuned than what a C compiler would guess ahead of time, because the JIT knows what your program actually does. the full story is in the interpreter and the JIT.

one language, many versions

a new java ships every march and every september, like a train that leaves on schedule whether your feature is on it or not. every two years, one release is LTS (long-term support): vendors keep patching it for years, so that's what most companies run. 17, 21 and now 25.

old versions called themselves 1.something: java 8 is "1.8" and its java -version still prints 1.8.0_.... from 9 on it's just the number. every release in detail lives in java 8 → 27.