Java basics: classes, main, and output

AP Computer Science A uses Java

• The AP CSA exam is written in Java.
• This site compiles Java with Java 8 language rules. Our standalone examples define a class with a main entry method. A helper class or an AP method answer does not need its own 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).
• Declarations such as int score = 10;, assignments, method calls and return statements end with a semicolon ;. An if, for or while block uses braces; do not add a semicolon just because its closing brace appears.

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

• Use ; after a declaration, assignment, method call or return; a block is not another such statement.
• For these Java 8 standalone programs, use public static void main(String[] args). The parameter name args can change, and String... args is also valid. Keep the entry method public, static, returning void and accepting a string array.

Now you try

• Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
• For a complete program, Run compiles and runs it. A method-only task has no Run button: use Check answer to compile it with the supplied harness and check the results.
• Your code compiles and runs on the server. Function tasks use the supplied harness to call your method.