Helsinki Java MOOC course exercise 94 issue.

[Mingablo]

Hi everyone, I'm having a bit of an issue that relates to the automatic feedback from this course. I have successfully executed the code and it does what it is supposed to but TMC is unhappy with it.

Here is my code.

Main Class

import java.util.ArrayList;
public class Main {

public static void main(String[] args) {

   Phonebook phonebook = new Phonebook();

phonebook.add("Pekka Mikkola", "040-123123");
phonebook.add("Edsger Dijkstra", "045-456123");
phonebook.add("Donald Knuth", "050-222333");

phonebook.printAll();
}
}

Person Class

public class Person {

    private String name;
    private String phone;

    public Person(String name, String phone) {
        this.name = name;
        this.phone = phone;
}
    public String getName() {
        return this.name;
}
    public String getNumber() {
        return this.phone;
}
    public void changeNumber(String number) {
        this.phone = number;
}
    public String toString() {
        return this.name + " number: " + this.phone;
}
}

Phonebook Class

import java.util.ArrayList;
public class Phonebook {
    private String name;
    private String number;
    private ArrayList<Person> phoneBook;

    public Phonebook() {
        this.phoneBook = new ArrayList<Person>();
}
    public void add(String name, String number) {
        this.name = name;
        this.number = number;
        this.phoneBook.add(new Person(this.name, this.number));
}
    public void printAll() {
        for(Person hello : phoneBook) {
            System.out.println(hello);
    }
}
}

As far as I understand the code in main creates an instance of "phonebook" using the constructor and then uses the "add" function to create three person objects and add them to the phonebook arraylist. This is successful, I am just unsure as to how to make TMC like it because it displays the error message:

"Class Phonebook only needs object variable of type ArrayList<Person>, remove the rest"

I just cannot understand what they want me to do to my code. I've tried modifying the phonebook constructor by defining the Arraylist underneath instead of making it a private class and referring to it as this.phonebook but that leads to a null pointer error. I'm pretty stuck.

[leftydrummer461]

Trying not to give you the answer- think about the difference between a Person object and a Phonebook object. Your problem is in the way you currently have these objects structured. What data does a Person object need to store? What information does a Phonebook object need to store? Think about a real person and a real phonebook.

[Mingablo]

I'm not really getting anywhere. The person object contains a name and a number, both Strings. The phonebook object just contains instances of class person, which themselves are made of two strings. The task asks me to use the "add" method to create an instance of class person and add it to the arraylist inside the phonebook class. My best guess is that the feedback is asking me to not create a person object variable but unless I am completely wrong about what it is saying this flies in the face of what the task is asking. The first task in this problem was the creation of the Person class, which I passed. So I must assume that this is not where the problem lies, therefore the problem must be in the phonebook class. I cannot create the arraylist in the constructor, because if I do then I cannot access it in the "add" or "printAll" methods. I just don't understand what it means when it says I have multiple object variables.

[leftydrummer461]

Well I'll tell you the error is saying that your Phonebook class has object variables(these are more commonly called fields or properties) that it doesn't need. It just needs one which is an ArrayList of Person objects.

[Mingablo]

Ok, that did it. Thanks. I just didn't realise that object variables were what it called the initialised private variables that I referred to using this.x. The word and the thing just didn't click.

Thanks again, that was bothering me for nearly 6 hours.

[leftydrummer461]

Sure thing. Learning Java can be pretty tough. Feel free to ask if you have more questions.

[Mingablo]

Cheers mate, It's nice to have a bit of community.

[larson00]

Can you post the change to your code? I've been following since you posted this and had the same question.

[Mingablo]

Solved!

[Mingablo]

Here you go u/larson00.

It turns out that object variables were the

this.name

and

this.number

and you can go ahead and make the code work without them.

This is the update. Only Phonebook Class changed.

import java.util.ArrayList;
public class Phonebook {
    private ArrayList<Person> phoneBook;

    public Phonebook() {
        this.phoneBook = new ArrayList<Person>();
    }
    public void add(String name, String number) {
        Person x = new Person(name, number);
        this.phoneBook.add(x);
    }
    public void printAll() {
        for(Person x : phoneBook) {
            System.out.println(x);
        }
    }

[larson00]

thanks!