尝试向对象数组添加元素时出现NullPointerException

我是一个java新手,我知道如何用C语言编程,所以当我试图编码时,我主要考虑用C语言。我想创建一个方法,允许我通过Book类型的数组将书籍添加到集合中。

class Set{
    private int maxElements;
    public static int countElements = 0;
    public Book arr[];

    public Set(int maxElements)
    {
        this.maxElements = maxElements;
    }

    public boolean addBook(Book b, Book[] arr)
    {
       
        countElements++;
        
        if(countElements > maxElements)
            return false;
        
        for(int i = 0; i < countElements; i++)
        {
            if(this.arr[i].equals(b) && this.arr[i] != null)
                return false;
        }
        
        arr[countElements - 1] = new Book(b.getTitle(), b.getAuthor());
        countElements++;
        return true;
    }

    public void printSet(Book arr[])
    {
        for(int i = 0; i < arr.length; i++)
        {
            System.out.println("Book ["+i+"] : "+arr[i].getTitle()+" | Author : "+arr[i].getAuthor());
        }
    }

    public String toString()
    {
        String res = null;
        for(int i = 0; i < arr.length; i++)
        {
            res = "Book ["+i+"] : "+arr[i].getTitle()+" | Author : "+arr[i].getAuthor();
        }
        return res;
    }

    public int getMaxElements()
    {
        return maxElements;
    }

}

Set类的主要方法是:

public class SetClient {
    public static void main(String[] args) {
        Set s = new Set(2);
       int size = s.getMaxElements();

        Book b1 = new Book("1984", "George Orwell");
        Book b2 = new Book("Hamlet", "William Shakespeare");
        Book[] arr = new Book[size];
        
        s.addBook(b1, arr);
        s.addBook(b2, arr);

        if(s.addBook(b1, arr) == true)
        {
            System.out.println("Added new book to the set: "+b1.getTitle());
        }
        else
        {
            System.out.println("Book NOT added.");
        }

        if(s.addBook(b2, arr) == true)
        {
            System.out.println("Added new book to the set: "+b2.getTitle());
        }
        else
        {
            System.out.println("Book NOT added.");
        }

        System.out.println(s);
    }
    
}

class Book{
    private int pages;
    private String title, author;

    public Book(int pages)
    {
        this.pages = pages;
    }

    public Book(String title, String author)
    {
        this.title = title;
        this.author = author;
    }

    public boolean equals(Object book)
    {
        if(book instanceof Book)
        {
            return ((Book)book).pages == this.pages && ((Book)book).title.equals(this.title);
        }
        return false;
    }

    public boolean equalsTitle(Object book)
    {
        if(book instanceof Book)
        {
            return ((Book)book).title == this.title;
        }
        return false;
    }

    public String toString()
    {
        return "This book has: "+pages+" pages";
    }

    public String getTitle()
    {
        return title;
    }

    public String getAuthor()
    {
        return author;
    }
}

NullPointerException when Creating an Array of objects 已经,但我仍然无法解决我的问题,我似乎不知道如何在我的案例中使用它。程序的输出是

Exception in thread "main" java.lang.NullPointerException: Cannot load from object array because "this.arr" is null
        at Set.addBook(Set.java:21)
        at SetClient.main(SetClient.java:10)

我应该换什么?