Library - now with searching!

Book.java

package library;

public class Book {

	private String title;
	private String isbn;
	private String author;
	private String category;
	
	public String getTitle() {
		return title;
	}

	public String getIsbn() {
		return isbn;
	}

	public String getAuthor() {
		return author;
	}

	public String getCategory() {
		return category;
	}

	public Book(String title, String isbn, String author, String category) {
		this.title = title;
		this.isbn = isbn;
		this.author = author;
		this.category = category;
	}
}

Library.java

package library;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class Library {

	Map<String, Book> booksByIsbn = new HashMap<String, Book>();
	// here I'm using the diamond operator
	Map<String, Set<Book>> booksByCategory = new HashMap<>();
	
	public void addBook(Book book) {
		booksByIsbn.put(book.getIsbn(), book);
		
		String category = book.getCategory();
		// if the category isn't already in the map
		if(!booksByCategory.containsKey(category)) {
			// put a new set into the map
			booksByCategory.put(category, new HashSet<>());
		}
		
		Set<Book> booksForCg = booksByCategory.get(category);
		booksForCg.add(book);
	}

	public Set<Book> findBooksInCategory(String category) {
		return booksByCategory.get(category);
	}

	public Set<Book> searchTitles(String substring) {
		Set<Book> matchingBooks = new HashSet<>();
		for(Book b: booksByIsbn.values()) {
			if(b.getTitle().contains(substring)) {
				matchingBooks.add(b);
			}
		}
		return matchingBooks;
	}
}

LibraryApp.java

package library;

import java.util.Set;

public class LibraryApp {

	public static void main(String[] args) {

		Library myLibrary = new Library();

		String title = "The Great Gatsby";
		String isbn = "178139685X";
		String author = "F. Scott Fitzgerald";
		String category = "Classics";

		Book gatsby = new Book(title, isbn, author, category);

		myLibrary.addBook(gatsby);

		myLibrary.addBook(new Book("To Kill A Mockingbird", "iMadeItUpIsbn", "Harper Lee", "Classics"));
		
		Set<Book> classicsBooks = myLibrary.findBooksInCategory("Classics");
		
		System.out.println("Classics:");
		for(Book b: classicsBooks) {
			System.out.println(b.getTitle());
		}
		
		System.out.println("Books whose titles contain 'Great':");
		Set<Book> booksWithGreat = myLibrary.searchTitles("Great");
		for(Book b: booksWithGreat) {
			System.out.println(b.getTitle());
		}
	}
}