Objects on the object's list

The list inside an object can hold objects of your own classes too. A bookshelf holds books, and each book has a title, an author and a number of pages. That makes a program of three files:

public class Main {
    public static void main(String[] args) {
        Bookshelf shelf = new Bookshelf("Living room");
        shelf.add(new Book("Emma", "Austen", 474));
        shelf.add(new Book("Persuasion", "Austen", 249));
        shelf.add(new Book("Dracula", "Stoker", 418));
        shelf.printAll();
        System.out.println("By Austen:");
        shelf.printBy("Austen");
    }
}

import java.util.ArrayList;

public class Bookshelf {
    private String room;
    private ArrayList<Book> books;

    public Bookshelf(String room) {
        this.room = room;
        this.books = new ArrayList<>();
    }

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

    public void printAll() {
        System.out.println(this.room + ":");
        for (Book book : this.books) {
            System.out.println(book);
        }
    }

    public void printBy(String author) {
        for (Book book : this.books) {
            if (book.getAuthor().equals(author)) {
                System.out.println(book);
            }
        }
    }
}

public class Book {
    private String title;
    private String author;
    private int pages;

    public Book(String title, String author, int pages) {
        this.title = title;
        this.author = author;
        this.pages = pages;
    }

    public String getTitle() {
        return this.title;
    }

    public String getAuthor() {
        return this.author;
    }

    public int getPages() {
        return this.pages;
    }

    public String toString() {
        return this.title + " by " + this.author + ", " + this.pages + " pages";
    }
}

Living room:
Emma by Austen, 474 pages
Persuasion by Austen, 249 pages
Dracula by Stoker, 418 pages
By Austen:
Emma by Austen, 474 pages
Persuasion by Austen, 249 pages

• The list's type names the class of its objects: ArrayList<Book>. Only Book objects can go on it.
• add(Book book) takes an object as its parameter (module 23) and puts it on the list.
• In the loops, book is one Book at a time, and the shelf uses it through Book's public methods. System.out.println(book) calls the book's toString, and book.getAuthor() its getter. Writing book.author in Bookshelf doesn't compile, because author is private to Book.
• Each class does its own job. Book knows how a book is written out, and Bookshelf knows which books it holds. main only makes the books and calls the shelf's methods.

The list holds references

The list doesn't hold copies of the books. Like any variable of a class type, each place on an ArrayList<Book> holds a reference to a Book object (module 22). So after

Book emma = new Book("Emma", "Austen", 474);
shelf.add(emma);

the variable emma and the shelf's list refer to the same Book. If Book had a method that changed the book, a change made through emma would show on the shelf too, and the other way around.

Making the object inside the method

A shelf could also take a book's values and make the Book itself. With module 21's overloading, both methods can be called add, and Java picks the one whose parameters match:

public void add(String title, String author, int pages) {
    this.books.add(new Book(title, author, pages));
}

Then shelf.add("Kindred", "Butler", 264); works too, and main never has to make a Book.