Java basics: classes, main, and output

AP Computer Science A uses Java

• The AP CSA exam is written in Java.
• Java is strict: every program lives inside a class, and starts from a method called main.
• You already think like a programmer — here we learn Java's rules.

The Java skeleton

• A tiny Java program looks like the example below. Your code goes inside main.
• System.out.println(...) prints one line of text (like Python's print).
• Every statement ends with a semicolon ;.

public class Main {
    public static void main(String[] args) {
        System.out.println("Java runs from inside main.");
        int year = 12;
        System.out.println("Grade " + year);
    }
}

Types

• Java needs a type for every variable: int (whole number), double (decimal), boolean (true/false), String (text).
• You write the type first: int score = 10;.
• Join text and values with +: "Grade " + year makes Grade 12.

Common mistakes

• Every statement ends with ;.
• main must be exactly public static void main(String[] args).

Now you try

• Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
• Press Run to compile and run, then Check answer.
• Your code compiles and runs on the server, so even the first run is fast.