Java how can I make a book class that can have one or more authors

Question

So I'm practictising Object oriented programming and I'm trying to make a Book class that can have multiple authors but I don't know how to do it. This is the UML of the excerise:

This is my author class which works fine:

public class Author {

    //attributen van de class auteur
    private String name;
    private String email;
    private char gender;

    //constructor
    public Author (String name, String email, char gender){
        this.name = name;
        this.email = email;
        this.gender= gender;
    }

    //methodes
    public String getName(){
        return name;
    }

    public String getEmail(){
        return email;
    }

    public void setEmail(String email){
        this.email = email;
    }

    public char getGender(){
        return gender;
    }

    //methode om gegevens van autheur opbject op te halen
    public String toString(){
        return "Author[name = " + name + ", email = " + email + ", gender = " + gender + "]";
    }
}

And here is the Book class that I tried to make:

public class Book {

    //attributes
    private String name;
    private Author authors [] = new Author[2];
    private double price;
    private int qty = 0;


    public Book(String name, Author authors[], double price, int qty) {
        this.name = name;
        authors[0] = new Author("Tan Ah Teck", "AhTeck@somewhere.com", 'm');
        authors[1] = new Author("Paul Tan", "Paul@nowhere.com", 'm');
        this.price = price;
        this.qty = qty;
    }


    public String getName() {
        return name;
    }

    public Author getAuthors() {
        return authors[authors.length];

    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public int getQty() {
        return qty;
    }

    public void setQty(int qty) {
        this.qty = qty;
    }

    public String toString() {
        return "Book [name = " + name + " authors = " + authors[0] + " email = " + authors[0].getEmail() + " price = " + price + " qty = " + qty + "]";
    }

    //methodes om gegevens van de autheur op te halen
    public String getAuthorNames() {
        return authors[].getName();
}

    public String getAuthorEmails() {
        return authors[].getEmail();
    }

    public char getAuthorGenders() {
        return authors[].getGender();
    }
}

So when I try to make an object of a book in my main.java the constructor of the book class is not working. Also at this function : public Author getAuthors() { it says: Array index is out of bounds. Also at the methods to get the author names, emails and genders it says: Unknown class authors[].

How can I modify this book class so a book can have one or more authors? (the Book class did work when a book only could have 1 author, but now I'm trying to change it so a book can have more authors)

Any kind of help is appreciated!

Answer 1

it says: Array index is out of bounds

First, it cannot say that with the given code, because it doesn't compile with the last three methods.

And well, authors.length == 2, and your array only has indicies 0 and 1

If you want to actually "get the authors", you want to return the array, not a specific one

public Author[] getAuthors() {
    return authors;
}

And if you did want to get one, then add the index.

It's also good habit to add boundary checking

public Author getAuthor(int index) {
    return (index < 0 || index >= authors.length) ? null : authors[index];
}

However, with these fixes, you might see null now because you actually have two list; the one you will pass to new Book(), and the field within the instance of that object.

Rather, you will want to do

private Authors[] authors;

public Book(String name, Author authors[], double price, int qty) {
    this.name = name;
    this.authors = authors;

And in the main method create the authors

Author[] authors = new Author[] {
    new Author("Tan Ah Teck", "AhTeck@somewhere.com", 'm'),
    new Author("Paul Tan", "Paul@nowhere.com", 'm')
};
Book b = new Book("name", authors, ...);
System.out.println(Arrays.toString(b.getAuthors()));

Also at the methods to get the author names, emails and genders it says: Unknown class authors[]

Because authors[].blah is not valid syntax. You need to loop over each of the array values to get the respective fields.

I would suggest Arrays.stream methods and StringJoiner for handling this

Answer 2

When you don't provide this keyword for authors within your constructor, the code works with your constructor parameter not with your class variable, therefore you encounter that problem.

try removing the authors from constructor like this:

public Book(String name, double price, int qty)