Filling an object array in Java

Question

So, I've recently switched to Java from C++ (due to my education) and doing some practice. I believe this question is not really smart, but I really need to know what am I doing wrong.

Simply, I have 3 classes:

mainClass
Composer
Concert

My problem is in the main():

public static void main(String[] args) {
    Composer Schubert = new Composer("Franz Schubert", "Classical Music", 6);
    Schubert.getConcert()[1].enterWholeConcertData(); //program just crashes here
}

Nothing goes after that, it just throws this exception:

Exception in thread "main" java.lang.NullPointerException at incredible_package.mainClass.main(mainClass.java:16)

NOTE: Everything works just fine, if I call the enterWholeConcertData() directly from Concert class object and it's not an array of them (like this):

Concert concert = new Concert();
concert.enterWholeConcertData();

Composer has a field, which is an array of Concert class and an int variable, that defines the number of elements in the Concert class (plus default constructor, another setters/getters, "show fields" method, "enter the whole info" method):

public class Composer{
        Composer(String name, String genre, int aNOC)
    {
        this.name = name;
        this.genre = genre;
        this.numberOfConcerts = aNOC;

        this.concert = new Concert[numberOfConcerts];
    }

    private int amountOfSpectators;
    private Concert[] concert;
}

Getter for concert in Composer class:

public Concert[] getConcert()
    {
        Concert[] copy = new Concert[this.concert.length];
        System.arraycopy(this.concert, 0, copy, 0, copy.length);
        return copy;
    }

Concert consists of (plus setters/getters, "show info" method, parameter constructor):

public class Concert{
    Concert()
    {
        date = "--.--.----";
        amountOfSpectators = 0;
    }

    private String date;
    private int amountOfSpectators;

    public void enterWholeConcertData()
        {
            System.out.println("Enter the date:");

            Scanner sDate = new Scanner(System.in);
            date = sDate.nextLine();
            sDate.close();

            System.out.println("Enter the number of spectators: "
                    + "");

            Scanner sAOW = new Scanner(System.in);
            amountOfSpectators = sAOW.nextInt();
            sAOW.close();
        }
}

Sorry, if I didn't mention anything important. You can ask me about that, I'll gladly add some new information.

A few comments: you should embrace Java conventions (camel case, variable start in small caps...) and unless necessary you should use collections instead of arrays (for example a List or a Set). — assylias
– assylias, Commented Feb 15, 2014 at 9:29
@assylias Thanks, I'll take the statement about conventions to my mind. And I was asked to do it with arrays. — Reinard
– Reinard, Commented Feb 15, 2014 at 9:32

Elfentech · Accepted Answer · 2014-02-15 09:33:08Z

3

From what I see:

public static void main(String[] args) {
    Composer Schubert = new Composer("Franz Schubert", "Classical Music", 6);
    Schubert.get_concert()[1].enterWholeConcertData(); //program just crashes here
}

You create a new Compser, setting its name, genre and NoC. You also initialize the array of 6 elements.

However, the elements in the array or Null, since you haven't initialize them. You should have at least done something like:

Shuber.get_concert()[1] = new Concert();

And same for any other items, before calling a method on.

answered Feb 15, 2014 at 9:33

Elfentech

7475 silver badges10 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Reinard Over a year ago

Uh-oh, haven't noticed I absolutely forgot about doing that in constructor. -Sigh- Thank you sincerely much. I knew my question wasn't smart enough.

Elfentech Over a year ago

Nevermind, when you are learning, all questions are smart enough ;)

Nigel Tufnel · Accepted Answer · 2014-02-15 09:33:20Z

0

You don't initialize any individual item of the Composer.concert array. In this case every item of Composer.concert array will be initialized to null and that's why you're getting NullPointerException.

Quoting JLS section 4.12.5:

Each array component is initialized with a default value when it is created

For all reference types the default value is null

You should initialize items of the Composer.concert array by yourself.

answered Feb 15, 2014 at 9:33

Nigel Tufnel

11.6k4 gold badges38 silver badges31 bronze badges

Collectives™ on Stack Overflow

Filling an object array in Java

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related