Greeter

Greeter.java

public class Greeter {
    public String greetMessage(User user) {
        return "Hi, "+user.email();
    }
}

GreetPrinter.java

public class GreetPrinter {
    public void static main(String[] args) {
        if (args.length < 1) {
            throw new RuntimeException("No email");
        }

        User user = new User(args[0]);
        Greeter greeter = new Greeter();
        System.out.println(greeter.greetMessage(user));
    }
}

User.java

public class User {
    private String email = "";

    public User(String email) {
        this.email = email;
    }

    public String email() {
        return email;
    }
}