Printing an object that holds a list

System.out.println(shelf) calls the shelf's toString (module 18). For an object that holds a list, the list belongs in that text too. The shortest way joins the list itself to the text:

public String toString() {
    return this.room + ": " + this.books;
}

The list writes itself out in brackets, with each book's own toString and commas between them: Hallway: [Emma by Austen, 474 pages, Dracula by Stoker, 418 pages]. That's hard to read when the books' texts have commas of their own. A loop gives you full control over the text.

Building the text in a loop

This toString starts with the room and the number of books, and then adds each book on a line of its own:

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));
        System.out.println(shelf);
        Bookshelf empty = new Bookshelf("Hallway");
        System.out.println(empty);
    }
}

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 String toString() {
        if (this.books.isEmpty()) {
            return this.room + ": no books";
        }
        String text = this.room + ": " + this.books.size() + " books";
        for (Book book : this.books) {
            text = text + "\n" + book;
        }
        return text;
    }
}

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

Living room: 3 books
Emma by Austen, 474 pages
Persuasion by Austen, 249 pages
Dracula by Stoker, 418 pages
Hallway: no books

• You met \n in module 20: inside a String, it's a line break. A String can hold several lines, and printing it prints each of them on its own line.
• text starts as the first line, and each round of the loop adds a line break and one book. + book calls the book's own toString.
• The line break goes before each book, not after it, so the text doesn't end with an empty line. println adds the last line break itself.
• An empty shelf gets its own text, checked first with isEmpty(). Without that check it would say Hallway: 0 books, which is fine too. What an empty object says is part of designing the class.

toString only builds the text and returns it; it doesn't print anything. The caller decides what to do with it: System.out.println(shelf) prints it, and "Found: " + shelf makes a longer String of it.