Comments & Friendly Bugs 🐛

Comments: notes for humans

Anything after // is a comment. Java ignores it completely. It's a note for people reading your code (including future you!).

public class Main {
    public static void main(String[] args) {
        // This robot greets the team
        System.out.println("Hi team!"); // says hi
        // System.out.println("This line is switched off");
    }
}

Only Hi team! is printed. The last line is "commented out", a handy way to switch code off without deleting it.

Java is picky about capitals

Java is case-sensitive: System and system are different words to Java.

system.out.println("Hi"); // ❌ Java doesn't know "system"
System.out.println("Hi"); // ✅

Bugs are normal (really!)

When Java finds a mistake, it stops and shows an error message before running anything. That isn't Java being mean; it's Java being a helpful robot inspector. 🔍

The most common beginner bugs:

💡

💡 Bug-hunting tip: read the error, look at the line number it mentions, then check the line just above it too.

🐞 Fun fact

In 1947, engineers found a real moth stuck inside a computer, causing problems. They taped it into their logbook as the "first actual case of bug being found". Computer bugs have been called bugs ever since!