All is right but getting error

public class ForumPost {
  private User mAuthor;
  private String mTitle;
  private String mDescription;

  public User getAuthor() {
    return mAuthor;
  }
  public ForumPost(User author,String title , String description){
   mAuthor=author;
   mTitle=title;
   mDescription=description;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getDescription() {
    return mDescription;
  }

  // TODO: We need to expose the description
}

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

  public String getLastName() {
   return mLastName; 
  }
}

public class Forum {
  private String mTopic;

  public String getTopic() {
      return mTopic;
  }

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

  public void addPost(ForumPost post) {

      System.out.printf("New post from %s %s about %s.\n",
                         post.getAuthor().getFirstName(),
                         post.getAuthor().getLastName(),
                         post.getTitle());

  }
}

public class Example {

  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("Ronit","Mankad");
    // Add the author, title and description
     ForumPost post = new ForumPost(author, "Feels","great");
     forum.addPost(post);
  }
}