如何使用类定义的数组并向其添加值（预定义的参数）？

Question

so what I did was create 3 classes: 1. Library Class (main) 2. Book Class (which I created an object instance of it on the main class) 3. LibraryException (just to handle exceptions)

However I am puzzled, I cannot figure out how to use the Array with the parameters / settings set in the Book class, here is my program:

public class Library
{
    String name;
    Book[] books;
    int nrBookz = 0;

    public Library(String name, int nrBookz) throws LibraryException {  
        if(nrBookz < 500){
            throw new LibraryException();
        }
        this.name = name;
        this.nrBookz = nrBookz;


    }


    public void addBook(Book Book) throws LibraryException {
        if(indexOf(Book) == -1){
            if(nrBookz < books.length){
                books[nrBookz++] = Book;
            }
        }
    }


    private int indexOf(Book Book)throws LibraryException {
        for(int i = 0; i < books.length; i++){
            if(books[i] == null){
                throw new LibraryException("That book does not exist!");
            }

            else if(books[i].equals(Book)){
                return i;
            }
        }
        return -1;
    }

    public static void main(String[]args) throws LibraryException{
        Library b = new Library("Sami Frasheri", 750);
        **b.addBook(new Book("Paul Colhen - Alchemist", 138));
        b.addBook(new Book("Paul Colhen - Winners Stand ALone", 139));
        b.addBook(new Book("Paul Colhen - The river Piedra I sat and I cried", 140));**
    }
}

.

public class Book {
    String name;
    int isbn;


    public Book(String e, int iNr) throws LibraryException{
        if(e == ""){
            throw new LibraryException("Book name is not allowed to be blank / empty!!");
        }

        else if(iNr < 1 || iNr > 9000){
            throw new LibraryException("The isbn is not within the allowed range (1 - 9000)! ");
        }
        name = e;
        isbn = iNr;
    }


    public boolean equals(Object obj){
        if(obj instanceof Book){
            Book p = (Book)obj;
            return name.toLowerCase().equals(p.getName().toLowerCase());
        }
        return false;
    }



    public String getName() {
        if(name == null || name == ""){
            System.out.print("name (Book) EXCEPTION TO BE ADDED!");
        }
        return name;
    }

    public void setname(String name) {
        this.name = name;
    }

    public int getIsbn() {
        if(isbn < 0){
            System.out.print("ISBN (Book) EXCEPTION TO BE ADDED!");
        }
        return isbn;
    }

    public void setIsbn(int isbn) {
        this.isbn = isbn;
    }

}

So my question is: How can I add (books) or make it possible to be added to the book array? (see blow - after I create the new Library (object of the class Library) *Marked with bold on the main method (Library class)

Answer 1

You are missing the array initialization :

public Library(String name, int nrBookz) throws LibraryException {  
    if(nrBookz < 500){
        throw new LibraryException();
    }
    this.name = name;
    this.nrBookz = nrBookz;
    this.books = new Book[nrBookz]; // add this

}