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Java Java Objects Delivering the MVP Forum

Johnathan Mullen
Johnathan Mullen
1,868 Points

Forum

My error Bummer! java.lang.NullPointerException (Look around Forum.java line 18)

Forum.java
public class Forum {
  private String topic;

  // TODO: add a constructor that accepts a topic and sets the private field topic

  public String getTopic() {
    return topic;
  }

  public Forum(String topic){
    topic = this.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()
    );
  }


}
User.java
public class User {
  // TODO: add private fields for firstName and lastName
  private String firstName;
  private String lastName;

  public User(String firstName, String lastName) {
    // TODO: set and add the private fields
    firstName = this.firstName;
    lastName = this.lastName;

  }

  // TODO: add getters for firstName and lastName
  public String getFirstName(){
  return firstName;
  }

  public String getLastName(){
  return lastName;
  }

}
ForumPost.java
public class ForumPost {
  private User author;
  private String title;
  private String description;

  // TODO: add a constructor that accepts the author, title and description

  public User getAuthor() {
    return author;
  }

  public String getTitle() {
    return title;
  }

  public String getDescription() {
    return description;
  }
  public ForumPost(User author, String title, String description){
   author = this.author;
    title = this.title;
    description = this.description;
  }
}
Main.java
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(args[0],args[1]);



    // TODO: initialize the forum post with the user created above and a title and description of your choice
    ForumPost post = new ForumPost(author,"Java","What I am trying to learn!");

    forum.addPost(post);



  }

}

1 Answer

Mihai Craciun
Mihai Craciun
13,520 Points

Be careful at constructors. It is : this.parameter = parameter;

I've pointed your mistakes in your code below

Forum.java

public class Forum {
  private String topic;

  // TODO: add a constructor that accepts a topic and sets the private field topic

  public String getTopic() {
    return topic;
  }

  public Forum(String topic){
    this.topic = topic   //<<<----------------------------------------------------- HERE
  }


  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()
    );
  }


}

User.java

public class User {
  // TODO: add private fields for firstName and lastName
  private String firstName;
  private String lastName;

  public User(String firstName, String lastName) {
    // TODO: set and add the private fields
    this.firstName = firstName;    //<--------------------------HERE
    this.lastName = lastName;    //<--------------------------HERE

  }

  // TODO: add getters for firstName and lastName
  public String getFirstName(){
  return firstName;
  }

  public String getLastName(){
  return lastName;
  }

}

ForumPost.java

public class ForumPost {
  private User author;
  private String title;
  private String description;

  // TODO: add a constructor that accepts the author, title and description

  public User getAuthor() {
    return author;
  }

  public String getTitle() {
    return title;
  }

  public String getDescription() {
    return description;
  }
  public ForumPost(User author, String title, String description){
    this.author = author;      //<--------------------------HERE
    this.title = title;            //<---------------------------HERE
    this.description = description; //<----------------HERE
  }
}