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Java Java Objects (Retired) Delivering the MVP Wrapping up

David Rojas
David Rojas
1,510 Points

Hi, I am in task 4 and I get this error:Bummer! java.lang.NullPointerException (Look around Forum.java line 13). Bug?

I have done everything I can, but when I uncomment and add the information about the book, I get is error:

Bummer! java.lang.NullPointerException (Look around Forum.java line 13)

is it a bug?

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

  public User getAuthor() {
    return mAuthor;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getDescription() {
    return mDescription;
  }

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

  }

  // TODO: We need to expose the description
}
User.java
public class User {
  private String mFirstName;
  private String mLastName;

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

  public void setFirstName(String firstName) {
       mFirstName = firstName;
  }

  public void setLastName(String lastName) {
       mLastName = lastName;
  }

  public String getFirstName() {
      return mFirstName;
  }

   public String getLastName() {
       return mLastName;
   }
}
Forum.java
public class Forum {
  private String mTopic;
  public Forum(String topic) {
  mTopic = topic;
}

  public String getTopic() {
      return mTopic;
  }

  public void addPost(ForumPost post) {
      System.out.printf("New post from %s %s about %s.\n",
                         post.getAuthor().getFirstName(),
                         post.getAuthor().getLastName(),
                         post.getTitle());
  }
}
Example.java
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");
    User author = new User(args[0], args[1]);
    // Add the author, title and description
    ForumPost post = new ForumPost(author, "Nikola Tesla", "Advanced Mechanics");
    forum.addPost(post);
  }
}

Not a bug, no - I've just got that challenge to work. Let me compare code to see where the difference is!

Steve.

1 Answer

Hi David - I think I see the issue. In the ForumPost constructor, you need to flip your assignment statements so the member variables are on the left (e.g. mDescription = description;). Currently, you're assigning the values of the member variables to the arguments, which don't exist outside of the constructor. Then, when you try to use the member variables later, the error occurs because they don't have values.

Well spotted!! :-)

David Rojas
David Rojas
1,510 Points

Hi Kathryn!

Thank you very much, I had my thoughts in reverse, and I was trying to understand why the machine had nowhere to point, I was going crazy with this!!! Thank you!!!