A Second file

In files that are not Main.java you can put other code, but only in the form of a class.

By this I mean, while in Main.java you are able to write something like this.

void sayHello() {
    IO.println("Hello");
}

void main() {
    sayHello();
}

In a file named Ball.java you need to put all code inside a Ball class.

class Ball {
    // You can write constructors, methods, fields, etc.
    final int size;

    Ball(int size) {
        this.size = size;
    }
}

// But you cannot have any "top level" methods or things outside
// of the Ball class

Then from Main.java you can make an instance of Ball

void main() {
    var ball = new Ball(10);
    IO.println("The ball is " + ball.size + "cm across");
}

When you run java src/Main.java it will find src/Ball.java and use the code in there.