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Java Java Objects Delivering the MVP Forum

Help with building Java constructor

I keep getting the error code "Make sure you add a new constructor to the Forum class that accepts a String named topic" but I have this in my code and feel like I'm missing something obvious? There are a couple of things I imagine could be wrong but I'm having a hard time because I don't feel like my error relates to what's actually the problem.

Forum.java
public class Forum {
  private String topic;

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

  private void setTopic(String topic) {
    private String topic = this.topic;
  }

  public String getTopic() {
    return topic;
  }



  /* Uncomment this when you are prompted to do so
  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

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

  }

  // TODO: add getters for firstName and 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;
  }
}
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);
    }
    /* Uncomment this when prompted
    Forum forum = new Forum("Java");
    // TODO: pass in the first name and last name that are in the args parameter
    User author = new User();
    // TODO: initialize the forum post with the user created above and a title and description of your choice
    ForumPost post = new ForumPost();
    forum.addPost(post);

// */
  }

}
Simon Coates
Simon Coates
28,694 Points

if you still haven't worked it out, the constructor code can be seen in one of the answers at https://teamtreehouse.com/community/javalangnullpointerexception-look-around-forumjava-line-17 . Also is an introduction to the problem of shadowing. If Kourosh Raeen's answer at this link is useful, please upvote. (i don't want to post code here unless you signal you want it)

Simon Coates
Simon Coates
28,694 Points

FYI, the setter method is messed up. Should probably look something like

private void setTopic(String topic){//could alternatively be public instead of private
    this.topic = topic;
}

The 'this' keyword is used to indicate the current object. In this instance, this.topic (read as 'this object's topic') differentiates from the topic parameter. The topic variable for the object is declared towards the top of the source code. ( I'm not sure it lets you call the setter from the constructor here)

1 Answer

A constructor is what you use when you create a new instance of an object. (ie: Forum forum = new Forum("Strings")) So it's asking you to create a constructor in the Forum class that defines what happens. Look at the User class for a hint

Thanks, this is really helpful. I'm new to Java and I googled "constructor" but what came up confused me somewhat --but I guess it's more similar to Python than I thought.

Most languages have quite a few similarities ?