Compiling and running

Python runs your source directly; Java takes two deliberate steps. Knowing them demystifies every error message you'll ever see.

The pipeline

Main.java  ──(javac)──▶  Main.class  ──(java)──▶  output
source           compiler        bytecode        JVM runs it

On your own machine it looks like:

javac Main.java    # compile: produces Main.class (bytecode)
java Main          # run: JVM executes the bytecode

• javac is the compiler: it checks your entire program and, only if everything is valid, emits .class bytecode files.
• java starts the JVM, which executes bytecode — translating it to fast machine code on the fly.

(Our playgrounds do both steps for you in the cloud — the pause before output is compilation.)

Two kinds of errors, two moments

Break it two ways and observe:

1. Delete the ; after int lessons = 13 → compile-time error. The compiler rejects it; nothing runs at all.
2. Change the print to System.out.println(10 / (lessons - 13)); → compiles fine, then runtime error (ArithmeticException: / by zero) while running.

Compile errors are grammar; runtime errors are logic. Java's design pushes as many mistakes as possible into the first, cheaper category.