Don't know why this doesn't work.

public class Forum {
  private String topic;

  // TODO: add a constructor that accepts a topic and sets the private field topic
  public void setTopic(String 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 void User(String firstName, String lastName) {
    // TODO: set and add the private fields
    firstName = "";
    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
  public void User(User author, String title, String description) {
    author = "";
    title = "";
    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(String getFirstName, String getLastName);
    // TODO: initialize the forum post with the user created above and a title and description of your choice
    ForumPost post = new ForumPost(getAuthor, getTitle, getDescription);
    forum.addPost(post);
  }

}