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Java Java Objects (Retired) Delivering the MVP Wrapping up

Jacob Schoeneman
Jacob Schoeneman
2,725 Points

Java Null Pointer Exception?

My wrapping up for some reason has a Null Pointer Exception and i don't know why?

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

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

  }

  public User getAuthor() {
    return mAuthor;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getDescription(){
   return 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) {
    // TODO:  Set the private fields here
    mFirstName = firstName;
    mLastName = lastName;

  }

  public String getFirstName(){
   return mFirstName; 
  }

  public String getLastName(){
    return mLastName;
  }

}
Forum.java
public class Forum {
  private String mTopic;

  public String getTopic() {
      return mTopic;
  }

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

  public void addPost(ForumPost post) {
      // When all is ready uncomment this...
      System.out.printf("New post from %s %s about %s \n and %s",
                         post.getAuthor().getFirstName(),
                         post.getAuthor().getLastName(),
                         post.getTitle(),
                         post.getDescription());

      }
}
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, "Finishing", "Finished Java Objects Course");
     forum.addPost(post);
  }
}

2 Answers

Kourosh Raeen
Kourosh Raeen
23,733 Points

The problem is in the ForumPost constructor. You need to assign the passed in author to mAuthor, which is already defined as private member variable, but you are redefining it:

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

In your constructor for ForumPost you have an extra User:

   public ForumPost(User author, String title, String description){
    //User mAuthor = author;  //can't have a type here
    mAuthor = author;
    mTitle = title;
    mDescription = description;

  }

With the User deleted it will work fine.