I'm really stuck

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 = topic; 
  }

 public Forum(String topic){
 topic=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 {
  private String firstName;
    private String lastName;
  // TODO: add private fields for firstName and lastName

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

  }
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");
    // TODO: pass in the first name and last name that are in the args parameter
    String firstName = args[0];
    String lastName = args[1];
    User author = new User(firstName, lastName);
    // TODO: initialize the forum post with the user created above and a title and description of your choice
    ForumPost post = new  ForumPost(author, "Programming is aweson");
    forum.addPost(post);

  }

}