I don't understand what I have to do in step 4 in the Main class... Can you please help me?

public class Forum {
  private String topic;

  // TODO: add a constructor that accepts a topic and sets the private field topic

  public String getTopic() {
    return topic;
  }

  public Forum (String topic){
    topic = getTopic;
  }
  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 {
  private String firstName;
  private String lastName;

  public User(String firstName, String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;

  }

  public String getFirstName (){
   return this.firstName;
  }

  public String getLastName(){
    return this.lastName;
  }

}

public class ForumPost {
  private User author;
  private String title;
  private String description;
  private String description;
  private String author;
  private String title;
  // TODO: add a constructor that accepts the author, title and description
public ForumPost(User author, String title, String description) {
  this.author = author; 
  this.title = title;
  this.descpription = 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");
    User.getFitstName;
    User.getLastName;
    User author = new User();


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

// */
  }

}