🔨 What does javac do?
javac is the Java compiler ‑ it takes the human‑readable code you just typed (HelloWorld.java) and translates it into a computer‑readable byte‑code file (HelloWorld.class).
- It checks your code for mistakes: if something is misspelled or out of place,
javacwill stop and tell you what line needs attention. - It optimises where it can so your program runs faster.
- The result is a compact, language‑neutral file that the Java Virtual Machine (JVM) can understand.
Think of
javaclike a master chef turning your recipe (source code) into a ready‑to‑cook meal (byte‑code).
🚀 What does java do?
java is the launcher that starts the Java Virtual Machine (JVM) and runs the byte‑code created by javac.
- It loads the
.classfile into the JVM. - The JVM interprets or Just‑In‑Time (JIT) compiles the byte‑code into machine instructions for your computer.
- It then executes those instructions step‑by‑step, doing whatever your program asks — in this case, printing
Hello, world!to the terminal.
Picture the JVM as a universal game console: any game (byte‑code) compiled by
javaccan play on any device that has the console (JVM) installed.
Why two steps?
- Write once: The compiler (
javac) produces platform‑independent byte‑code. - Run anywhere: The JVM (
java) makes that byte‑code work on Windows, macOS, Linux, or even a Raspberry Pi — no changes required!
Quick recap
| Command | Role | Typical output |
|---|---|---|
javac HelloWorld.java | Compiles source to byte‑code | HelloWorld.class |
java HelloWorld | Launches JVM to execute byte‑code | Hello, world! |
Now you know what happens under the hood every time you hit compile & run. đźš—đź’¨