I'm not sure what I've done wrong. please help!

public class Forum {
  private String topic;

  public Forum(String topic){
   topic = topic; 
  }

  public String getTopic() {
      return topic;
  }

  public void addPost(ForumPost post) {
      // When all is ready uncomment this...
      System.out.printf("New post from %s %s about %s \n and %s",
                         post.getAuthor().getFirstName(),
                         post.getAuthor().getLastName(),
                         post.getTitle(),
                         post.getDescription());


  }
}

public class User {
    private String firstName;
  private String lastName;

  public User(String firstName, String lastName) {
    // TODO:  Set the private fields here
    firstName =firstName;
    lastName = lastName;
  }
  public String getFirstName(){
    return firstName;
  }
  public String getLastName(){
    return lastName;
  }
}

public class ForumPost {
  private User author;
  private String title;
  private String description;
  public ForumPost(User author, String title, String description){
    author = author;
    title = title;
    title = " ";
    description = description;
    description = " "; 
  }  

  public User getAuthor() {
    return author;
  }

  public String getTitle() {
    return title;
  }

  // TODO: We need to expose the description
  public String getDescription(){
    return description;
  }
}

public class Main {

  public static void main(String[] args) {
    System.out.println("Starting forum example...");
    if (args.length < 2) {
       System.out.println("first and last name are required. eg:  java Example Craig Dennis");
    }
   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,"Finishing OOP","It feels Great!" );
    forum.addPost(post);
  }
}