用Java填充对象数组

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.

Answer 1

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.

Answer 2

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.