package com.springmvc.repository.impl;
...
import com.springmvc.exception.BookIdException;

@Repository
public class BookRepositoryImpl implements BookRepository {
    ...
    public Book getBookById(String bookId) {
        Book bookInfo = null;
        for (int i = 0; i < listOfBooks.size(); i++) {
            Book book = listOfBooks.get(i); 
            if (book != null && book.getBookId() != null && book.getBookId().equals(bookId)) {
                bookInfo = book;
                break;
            }
        }

➊

        if (bookInfo == null)
            throw new BookIdException(bookId);

        return bookInfo;
    }
    public void setNewBook(Book book) {
        listOfBooks.add(book);
    }
}