User class

Greeter.java

public class Greeter {

  public String greetMessage(User user) {
    if(StringUtils.isEmpty(user.email())) {
      return null;
    }
    return "Hi, "+user.email();
  }

}

GreetPrinter.java

public class GreetPrinter {
  public void static main(String[] args) {
    User user = new User();
    if (args.length > 1) {
      user.setEmail(args[0]);
    }
    Greeter greeter = new Greeter();
    String msg = greeter.greetMessage(user);
    if (msg != null) {
      System.out.println(msg);
    }
  }
}

User.java

public class User {
  private String email;

  public void setEmail(String email) {
    this.email = email;
  }

  public String email() {
    return email;
  }

}