What Java is, and your first program

How Java actually runs

Java is built around one promise: write once, run anywhere. You write .java source files in plain text. The compiler javac turns them into bytecode (.class files) — not machine code for your specific computer, but instructions for the JVM (Java Virtual Machine). A JVM exists for Windows, macOS, and Linux, and each one runs that same bytecode.

So the loop is: write → compile → run.

The write-compile-run loop: one source file, one bytecode file, any machine.

Your first program

Create a file named Hello.java:

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Reading it line by line:

• public class Hello — Java code lives inside classes. The file name must match the public class name, so this file must be Hello.java.
• public static void main(String[] args) — the entry point. When you run the program, the JVM looks for exactly this method and starts there.
• System.out.println(...) — prints a line of text to the console.

Compile and run from a terminal in that folder:

javac Hello.java
java Hello

Output
Hello, Java!

Note it is java Hello, not java Hello.class — you name the class, not the file.

A class file is a recipe card and the JVM is the cook. You hand over the recipe's name, not the card's filename.

One modern aside: for a single file, java Hello.java also works — the JVM compiles it in memory. We teach the two-step loop because every real project uses it.

Try it

• run it: create Hello.java, compile it, run it, see the greeting.
• tweak it: change the message to your own name. Predict which of the two commands you must repeat, then check.
• break it: rename the class to Helo but leave the file name alone, then compile. Read the error slowly — it names the exact problem.
• build it: print three lines about yourself using three println calls.