The Anonymous Main Class

You are only allowed to make an anonymous main class inside the default package.

// Allowed
void main() {
    System.out.println("Hello, world");
}

// Not Allowed
package myprogram;

void main() {
    System.out.println("Hello, world");
}

This means that for classes in packages you have to wrap them in an explicitly named class like everything else.

// Allowed
package myprogram;

class Main {
    void main() {
        System.out.println("Hello, world");
    }
}