Arraylist inside an Hashmap JAVA - arraylist

Question

I'm trying to make a booking app. I have Hotel, Client, and Book classes. Every client has a unique number, same for Book, but a single client can have 0 or many booked rooms.
I started with the idea of using an HashMap since I'm starting learning Java and that might be useful on situations like this.
On the Hotel class the HashMap was initialized this way:
private HashMap<Integer, ArrayList<Book>> hm
(the key is the client number and values is the ArrayList, an open possibility to every client to book more than one room, but each ArrayList has to belong to a single client)
now and want to book a room, and put it on the hashMap, but I'm stucked since I've no idea how to inicialize a ArrayList for this specific client and don't be used in any other situation.This is what I got so far inside my class hotel inside a funtion in which I intend to book..
Book b=new Book(new GregorianCalendar(2018,month,day),days);
System.out.println("Do you have a client number?(y/n");
if (sc.next().toLowerCase().equals("y")) {
System.out.println("Insert your client number");
int n =sc.nextInt();
for (Client c:this.clients) {
if (c.getCodC()==n) {
this.hm.put(c.getCod(),???)
My question is, how can I create and manipulate an ArrayList and be sure that's the right one for each client?

Answer 1

Try out below code after your last if statement:
List<Book> books = hm.get(n);
if(books== null) {
books = new ArrayList<>();
}
books.add(b);
hm.put(n, books);