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Java Java Objects (Retired) Delivering the MVP Wrapping up

Patrick Hebert
Patrick Hebert
2,254 Points

Totally stuck on wrap up question... null pointer exception

I'm stuck - can't figure out why I'm getting this null pointer exception error...

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 ForumPost(User author, String title, String description) {

  }

  public String getDescription(){
    return mDescription;
  }

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

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


  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) {
      /* When all is ready uncomment this... */
      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");
    // 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, "Java", "Desc");
    forum.addPost(post);
  }
}

You need to add your variables to your constructor in class ForumPost:

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

I always put my constructor just below the instance variable declarations also.

3 Answers

Kourosh Raeen
Kourosh Raeen
23,733 Points

Your ForumPost constructor is empty. You need to set the member variables in there.

Stephen Shine
Stephen Shine
5,991 Points

Agree with what's been said. Don't forget that your constructor takes input arguments, but then needs to assign those inputs to the member variables.

You're getting the null pointer exception because your constructor doesn't know what to do with the input you're giving it.

Patrick Hebert
Patrick Hebert
2,254 Points

Thanks everyone!! Don't know how I missed that...