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Start your free trialWilliam Mariadason
1,393 PointsWhy do I get a NullPointerException here?
Beginning forum example java.lang.NullPointerException at Forum.addPost(Forum.java:17) at Main.main(Main.java:15) at JavaTester.run(JavaTester.java:185) at JavaTester.main(JavaTester.java:45)
I don't understand why I get this
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()
);
}
}
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;
}
public String getFirstName(){
return firstName;
}
public String getLastName(){
return lastName;
}
// TODO: add getters for firstName and lastName
}
public class ForumPost {
private User author;
private String title;
private String description;
// TODO: add a constructor that accepts the author, title and 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;
}
}
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, "title", "description");
forum.addPost(post);
}
}
1 Answer
andren
28,558 PointsBecause you are setting topic (the parameter) equal to topic the field variable (which has not been initialized with any data) in other words you have the assignments backwards. If you reverse the order like this:
this.topic = topic;
That should clear the null-pointer error.
Edit:
I notice that you are doing the same thing in all of the other constructors, so you will have to reverse the order in the other constructors as well.