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Java Java Objects (Retired) Delivering the MVP Wrapping up

Posted by Unsubscribed User
Unsubscribed User
5,777 Points

NullPointerException error...

I'm trying to understand the logic behind why I'm getting this error. This is how I understand this program to work:

Example.java
public class Example {

  public static void main(String[] args) { //takes an array of strings args at the command
    System.out.println("Starting forum example...");
    if (args.length < 2) { //this makes you input your name at the command
       System.out.println("first and last name are required. eg:  java Example Craig Dennis");
    }
    Forum forum = new Forum("Java"); // we start a new object forum with topic "Java"
    User author = new User(args[0], args[1]); //this takes the strings at positions 0 and 1
    // that were entered as args at the command and puts them in a User object author
    ForumPost post = new ForumPost(author, "Please be done", "This is really awesome if it works" );  
//this throws all these inputs into a post object of type ForumPost
    forum.addPost(post); //this takes the post object and tosses it into the addPost method 
// which uses the inputs from post to print out "New post from %s %s about %s.\n"
  }
}

I'm getting a NullPointerException here in the Forum.java file:

  public void addPost(ForumPost post) {
      System.out.printf("New post from %s %s about %s.\n",
                         post.getAuthor().getFirstName(), // this line here
                         post.getAuthor().getLastName(),
                         post.getTitle());
  }

Is the method getAuthor(). accessing a null object? If so, how do I make it not null...

Jeff Wilton
Jeff Wilton
16,646 Points
Jeff Wilton
Jeff Wilton
16,646 Points

I can't say for certain without seeing all of your code and the actual code you are using to start your program, but I can explain a few things to you:

1) You are getting a nullPointerException because you are trying to call a getter on a null object. On the line you indicated, you are calling two getters, so there are two possibilities for null objects: either the Author (aka User) is null, or the ForumPost itself is null. In the typical programming world it is bad practice to call a getter on an object that has the possibility of being null without first checking to see if it is null:

if(post != null && post.getAuthor() != null) {
  //do the rest of the work
}
else {
  //something went wrong!
}

2) The code you posted looks OK, so the problem might be in your User or Forum classes.

Mine look like this:

public class User {

  private String mFirstName;
  private String mLastName;    

  public User(final String firstName, final String lastName) {
    this.mFirstName = firstName;
    this.mLastName = lastName;
  }  

  public String getFirstName() {
    return mFirstName;
  }  

  public void setFirstName(final String firstName) {
    this.mFirstName = firstName;
  }

  public String getLastName() {
    return mLastName;
  }  

  public void setLastName(final String lastName) {
    this.mLastName = lastName;
  }
}

public class ForumPost {

  private User mAuthor;
  private String mTitle;
  private String mDescription;  

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

  public User getAuthor() {
    return mAuthor;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getDescription() {
    return mDescription;
  }
}

Please note, using the 'this' keyword in the constructors is not necessary in this case because the variable names are different (firstName vs mFirstName), but it is a good programming practice. Same thing where I am using the final keyword before my arguments.. just a good programming practice to make sure you don't try to change the argument value to something else inside the method.

3) your code is Example.java looks good to me. I am able to run it with my other classes and get the expected result. :)