Library.java

import java.util.ArrayList;
public class Library {

    private ArrayList<Book> books;
    private ArrayList<Customer> customers;
    private ArrayList<Membership> memberships;


    // Generating constructor, getters and setters.
    public Library() {
        books = new ArrayList<>();
        customers = new ArrayList<>();
        memberships = new ArrayList<>();

    }

    public ArrayList<Book> getBooks() {
        return books;
    }

    public ArrayList<Customer> getCustomers() {
        return customers;
    }

    public ArrayList<Membership> getMemberships() {
        return memberships;
    }


    public void setBooks(ArrayList<Book> books) {
        this.books = books;
    }

    public void setCustomers(ArrayList<Customer> customers) {
        this.customers = customers;
    }

    public void setMemberships(ArrayList<Membership> memberships) {
        this.memberships = memberships;
    }


    // Adding methods

    public void addBook(Book book) {
        books.add(book);
    }

    public void addCustomer(Customer customer) {
        customers.add(customer);
    }

    public void addMembership(Membership membership) {
        memberships.add(membership);
    }

    public void displayBooks() {
        for (Book book : books) {
            System.out.println(book.getTitle());
        }
    }

    public void displayCustomers() {
        for (Customer customer : customers) {
            System.out.println(
                    customer.getFirstName() + " " +
                            customer.getLastName()
            );
        }
    }
}