我觉得很慷慨，所以我修正了你的代码。所有的编译错误都消失了，还修正了一些逻辑错误。

public class MyBookstore {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        Bookstore myBkStore = new Bookstore();

        int y = 0;
        int user_choice = 2;
        boolean quit = false;

        do {
            // display menu to user
            // ask user for his choice and validate it (make sure it is between 1 and 6)
            System.out.println();
            System.out.println("1) Add a book to the stock");
            System.out.println("2) Sell a book in stock");
            System.out.println("3) List the titles of all the books in stock (in the Bookstore object)");
            System.out.println("4) List all the information about the books in stock (in the Bookstore object)");
            System.out.println("5) Print out the gross income of the bookstore");
            System.out.println("6) Quit");
            System.out.println();
            System.out.print("Enter choice [1-6]: ");
            user_choice = s.nextInt();
            s.nextLine(); // THIS IS NEEDED TO MOVE THE SCANNER TO THE NEXT LINE BEFORE GETTING THE TITLE.
            switch (user_choice) {
            case 1:
                System.out.print("Enter a book title: ");
                String bt = s.nextLine(); // stores title of book user enters in
                if (myBkStore.inStock(bt)) {
                    System.out.print("How many more to add to the stock: ");
                    y = s.nextInt();
                    myBkStore.addBookQuantity(bt, y);
                } else {
                    System.out.println("Enter the amount of pages of the book: ");
                    int pages = s.nextInt();
                    System.out.println("Enter the price of the book: ");
                    double price = s.nextDouble();
                    System.out.println("Enter the quantity to add: ");
                    int quant = s.nextInt();
                    // myBkStore.Book(bt, pages, price, quant);
                    myBkStore.addNewBook(new Book(bt, pages, price, quant));
                }

                break;
            case 2:
                System.out.println("Enter book title to buy: ");
                String bookT = s.nextLine();
                myBkStore.sellBook(bookT);
                break;
            case 3:
                myBkStore.listTitles();
                break;
            case 4:
                myBkStore.listBooks();
                break;
            case 5:
                System.out.println("\nStore Gross income: " +  myBkStore.getIncome());
                break;
            case 6:
                System.out.println("Thanks for coming");
                quit = true;
                break;
            default:
                System.out.println("\nInvalid Choice");
            }
        } while (!quit);
        s.close();
    }

    static class Bookstore {
        private Book[] books; // all the books in this bookstore
        private int totalBooks; // the number of books in this bookstore
        private double grossIncome; // the gross income of the bookstore (will be incremented when books are sold)

        // Constructor: A new Bank object initially doesn’t contain any accounts.
        public Bookstore() {
            books = new Book[100];
            totalBooks = 0;
            grossIncome = 0;
        }

        // Creates a new bank account using the customer name and the opening balance
        // given as parameters
        // and returns the account number of this new account. It also adds this account
        // into the account list
        // of the Bank calling object.
        public void addNewBook(Book b) {
            if (totalBooks < books.length) {
                books[totalBooks] = b;
                totalBooks++;
            } else {
                System.out.println("\nSorry, cannot add book to stock.");
            }
        }

        public void addBookQuantity(String title, int quantity) {
            for (int i = 0; i < totalBooks; i++) {
                if (title == books[i].getTitle()) {
                    books[i].addQuantity(quantity);
                    System.out.println("Quantity added successfully");
                    return;
                }
            }
        }

        public boolean inStock(String title) {
            for (int i = 0; i < totalBooks; i++) {
                if (title.equals(books[i].getTitle())) {
                    if ( books[i].getQuantity() > 0) {
                        return true;
                    } else {
                        return false;
                    }
                }
            }
            return false;
        }

        public boolean sellBook(String title) {
            int i;
            boolean sellflag = false;

            // Checks to see if the books are in stock.
            boolean retval = inStock(title);

            // If so, completes the sale.
            if (retval) {
                for (i = 0; i < totalBooks && !sellflag; i++) {
                    if (title.equals(books[i].getTitle())) {
                        int quantity = books[i].subtractQuantity();
                        grossIncome += (books[i].getPrice()) * quantity;
                        sellflag = true;
                    }
                }
            }
            return retval;
        }

        public void listTitles() {
            for (int i = 0; i < totalBooks; i++) {
                System.out.println(books[i].getTitle());
            }
        }

        public void listBooks() {
            int i;

            System.out.println("\nList of Books\n======");
            for (i = 0; i < totalBooks; i++) {
                System.out.println(books[i]);
            }
            System.out.println();
        }

        public double getIncome() {
            return grossIncome;
        }
    }

    private static class Book {

        private String title;
        private int numOfPages;
        private double price;
        private int quantity;

        public String toString() {
            return "Title: " + title + "\nNumber of pages: " + numOfPages + "\nPrice:" + price + "\nQuantity: "
                    + quantity + "\n";
        }

        public Book(String thetitle, int pages, double cost, int num) {
            title = thetitle;
            numOfPages = pages;
            price = cost;
            quantity = num;
        }

        public String getTitle() {
            return title;
        }

        public double getPrice() {
            return price;
        }

        public int getQuantity() {
            return quantity;
        }

        public void addQuantity(int amount) {
            quantity = quantity + amount;

        }

        public int subtractQuantity() {
            System.out.println("Amount to buy");
            Scanner s = new Scanner(System.in);
            int amount = s.nextInt();
            quantity = quantity - amount;
            return amount;
        }
    }
}

示例输出：

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 1
Enter a book title: Effective Java
Enter the amount of pages of the book: 
300
Enter the price of the book: 
29.99
Enter the quantity to add: 
10

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 1
Enter a book title: Learn Python
Enter the amount of pages of the book: 
100
Enter the price of the book: 
10.00
Enter the quantity to add: 
5

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 2
Enter book title to buy: 
Effective Java
Amount to buy
1

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 3
Effective Java
Learn Python

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 4

List of Books
======
Title: Effective Java
Number of pages: 300
Price:29.99
Quantity: 9

Title: Learn Python
Number of pages: 100
Price:10.0
Quantity: 5

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 5

Store Gross income: 29.99

1) Add a book to the stock
2) Sell a book in stock
3) List the titles of all the books in stock (in the Bookstore object)
4) List all the information about the books in stock (in the Bookstore object)
5) Print out the gross income of the bookstore
6) Quit

Enter choice [1-6]: 6
Thanks for coming

您的主要问题与Scanner的使用有关。
1.对于标题，因为它们可以包含多个单词（用空格分隔），所以需要使用Scanner#nextLine()而不是Scanner#next()。
1.当你在主菜单中输入数字时，你需要占用这一行。调用Scanner#nextInt()并不会占用这一行。因此，你需要在下一个提示之前调用Scanner#nextLine()，这样扫描程序就会移动到下一行，以获取下一个输入的值。