I have no idea how to do the last part of this challenge.

public class Forum {
  private String topic;

  // TODO: add a constructor that accepts a topic and sets the private field topic
public Forum(String topic){
    this.topic=topic;
}
  public String getTopic() {
    return topic;
  }



  public void addPost(ForumPost post) {
    System.out.printf("A new post in %s topic from %s %s about %s is available",
            topic,
            post.getAuthor().getFirstName(),
            post.getAuthor().getLastName(),
            post.getTitle()
    );
  }


}

public class User {
  // TODO: add private fields for firstName and lastName
private String firstName;
private String lastName;

  public User(String firstName, String lastName) {
    // TODO: set and add the private fields
    this.firstName=firstName;
    this.lastName=lastName;
  }

  // TODO: add getters for firstName and lastName
public String getFirstName(){
    return firstName;
}
  public String getLastName(){
    return lastName;
}
}

public class ForumPost {
  private User author;
  private String title;
  private String description;

  // TODO: add a constructor that accepts the author, title and description
private ForumPost(User author,String title,String description){
    this.author=author;
    this.title=title;
    this.description=description;
}
  public User getAuthor() {
    return author;
  }

  public String getTitle() {
    return title;
  }

  public String getDescription() {
    return description;
  }
}

public class Main {

  public static void main(String[] args) {
    System.out.println("Beginning forum example");
    if (args.length < 2) {
      System.out.println("Usage: java Main <first name> <last name>");
      System.err.println("<first name> and <last name> are required");
      System.exit(1);
    }

    Forum forum = new Forum("Java");
    // TODO: pass in the first name and last name that are in the args parameter

    User author = new User();
    // TODO: initialize the forum post with the user created above and a title and description of your choice
   this.lastName=lastName;

    ForumPost post = new ForumPost();
    forum.addPost(post);


  }

}