java.lang.NullPointerException

Question

Hey all. When I tried compiling this code, it brought up this:

Beginning forum example java.lang.NullPointerException at Forum.addPost(Forum.java:17) at Main.main(Main.java:16) at JavaTester.run(JavaTester.java:185) at JavaTester.main(JavaTester.java:45)

I looked at line 17 in Forum, but I can't figure out what's wrong. Does anyone have any ideas?

Forum.java

public class Forum {
  private String topic;

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

  public String getTopic() {
    return 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 {
  private String firstName;
  private String lastName;

  public User(String firstName, String lastName) {
    firstName = this.firstName;
    lastName = this.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;

  public ForumPost(User author, String title, String description){
    author = this.author;
    title = this.title;
    description = this.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);
    }

    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, "The Poverty of Historicism", "Philosophy/Sociology");
    forum.addPost(post);


  }

}

Answer 1 · 2017-05-23T00:05:32Z

May 23, 2017 12:05am

In your constructors you have things a bit backwards:

  public ForumPost(User author, String title, String description){
    author = this.author;
    title = this.title;
    description = this.description;
  }

Instead of setting the member variables, the this.whatever you are wiping out the local parameters (all the members are null at this point). What you should be doing is:

  public ForumPost(User author, String title, String description){
    this.author = author;
    this.title = title;
    this.description = description;
  }

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Kaytlynn Ransom

Kaytlynn Ransom

java.lang.NullPointerException

1 Answer

Seth Kroger

Seth Kroger