HEEELP. I don't know what's my problem...

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 the private fields here
    firstName = this.firstName;
    lastName = this.lastName;    
  }

  public String getFirstName() {

    return firstName;
  }

  public String getLastName() {

    return lastName;
  }

  }

  // TODO: add getters for firstName and lastName

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

  // TODO: add a constructor that accepts the author, title and description
public ForumPost(User author, String title, String description){
    this.author = author; //<--- your code will be different here
    this.title = title; // <---- and here
    this.description = description; //<---- aaand here 

  }
  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");



    //Take the first two elements passed args

  User author = new User(args[0], args[1]);

    //Add the author, title and description
    ForumPost post = new ForumPost(author, "A Title", "A Description");
    forum.addPost(post);

  }

}