public void addBook(int x, String title, String author, int year, String isbn, double price, String subject, String wisbn, int numProblems){
       if (x == 1){
           Workbook newWBook = new Workbook (title, author, year, isbn, price, numProblems);
           bookMap.put(newWBook.hashCode(), newWBook);
       }
       else if (x == 2){
           Textbook newTBook = new Textbook (title, author, year, isbn, price, wisbn, subject);
           bookMap.put(newTBook.hashCode(), newTBook);
       }
       else if (x == 3 ){
           Textbook newBook = new Textbook (title, author, year, isbn, price, wisbn, subject);
           bookMap.put(newBook.hashCode(), newBook);
       }

   }