Thread: [Beginner] How to add a new object to an object array?

I'm a complete newbie to Java and I want to create a class called Set, where the maximum number of elements of the set is given as parameter in the constructor and the elements of the set are stored in an array of type Book[]. The elements shouldn't exceed the maximum capacity and they should be unique.

class Set{
    private int maxElements;
    public static int countElements = 0;
    public Book[] arr = new Book[countElements];  
 
    public Set(int maxElements)
    {
        this.maxElements = maxElements;
    }
 
    public boolean addBook(Book b)
    {
        int i = 0, j = 0;
 
      //  if(countElements > maxElements)
        //    return false;
 
        /*for(i = 0; i < countElements; i++)
        {
            if(this.arr[i].getTitle().equals(b.getTitle()))
                return false;
        }*/
 
        Book[] newArr = new Book[countElements+1];
 
        if (this.arr[0] == null) // if the array is empty
            newArr[0] = this.arr[0];
        else
            for(i = 0; i < countElements; i++)
            {
                newArr[i] = this.arr[i];
            }
        newArr[countElements] = new Book(b.getTitle(), b.getAuthor());
        countElements++;
 
        // here I want to somehow also return newArr or have newArr replace this.arr as the field
 
        return true;
    }
 
    public void printSet(Book arr[])
    {
        if(arr[0] == null)
        {
            System.out.println("No books in the set.");
        }
        else
        {
            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;
    }
 
}

public class SetClient {
    public static void main(String[] args) {
        Set s = new Set(2);
 
 
        Book b1 = new Book("1984", "George Orwell");
        Book b2 = new Book("Hamlet", "William Shakespeare");
 
        s.addBook(b1);
        s.addBook(b2);
 
        /*if(s.addBook(b1) == true)
        {
            System.out.println("Added new book to the set: "+b1.getTitle());
        }
        else
        {
            System.out.println("Book NOT added.");
        }
 
        if(s.addBook(b2) == 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;
    }
}

I know this can be solved easier with a list, but I need to use an array. I've changed the addbook and my overall code countless times but I'm never able to add even 1 book. I've looked up a lot of sources and they all make sense but somehow I can't implement a decent solution. What should I change? Currently, I get the following message when running the code:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
at Set.addBook(Set.java:26)
at SetClient.main(SetClient.java:9)

after start arr.length is 0, then can't do
if (arr[0] == null)
because [0] doesn't exist

countElements can't be static in situation of two sets with different length

public boolean addBook(Book b) {
    Book[] newArr = new Book[countElements +1];
 
    for (int i=0; i < countElements; i++)
         newArr[i] = this.arr[i];
 
    newArr[countElements] = b;
    arr = newArr;
    countElements++;
 
    return true;
  }