Methods and input

A method is a named piece of a program. You have used main and println; now write your own. A method declares what it takes (its parameters) and what it gives back (its return type):

public class Squares {
    static int square(int n) {
        return n * n;
    }

    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            System.out.println(i + " squared is " + square(i));
        }
    }
}

static int square(int n) takes one int, called n inside the method, and returns an int. A method that returns nothing has the return type void, like main.

Reading input

Scanner reads what the user types (the program's standard input). in.nextInt() reads the next whole number; in.hasNextInt() tells whether there is one. In a lesson block the input can be given in advance, as in the next exercise; in your own programs, type into the output area while the program runs.

import java.util.Scanner;

public class Sum {
    static int sum(Scanner in) {
        int total = 0;
        // read every number: while (in.hasNextInt()) { … }
        return total;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("Sum: " + sum(in));
    }
}

Two files

Bigger programs are split into several classes, one per file. Here Main uses a method of the class Greeter, which lives in Greeter.java; javac compiles both. In this block you can also add files (+), rename them (double-click a tab) and delete them (×).

public class Main {
    public static void main(String[] args) {
        System.out.println(Greeter.greet("Ada"));
        System.out.println(Greeter.greet("Grace"));
    }
}

That is the end of the first five lessons. Everything you wrote is in /home/student in this browser; the Environment panel downloads it as a .zip.