More AP-style questions

Practice.java

public class Book
{
	private String title;
	private String author;
	private String genre;
	private int numberOfPages;

	/* Other methods not shown. */

	public String getTitle() {
		/* Implementation not shown. */
	}

	public String getGenre() {
		/* Implementation not shown. */
	}

	public String getAuthor() {
		/* Implementation not shown. */
	}

	public int getNumberOfPages() {
		/* Implementation not shown. */
	}
}

public class Library
{
	private ArrayList<Book> books;

	/* Other methods not shown. */

	/* Returns the average number of pages in books
	 * in this library. 
	 */
	public int averageNumberOfPages() {
		// TODO: implement
		return 0;
	}

	/* Returns the author of the book with the given
	 * title.
	 *
	 * @param title - the title.
	 */
	public String getAuthorForBook(String title) {
		// TODO: implement
		return null;
	}

	/* Returns all books in this library that are
	 * written by the given author.
	 *
	 * @param author - the author.
	 */
	public ArrayList<Book> getBooksByAuthor(String author) {
		// TODO: implement
		return null;
	}

	/* Returns the title of the longest book (in pages) 
	 * with the given genre.
	 *
	 * @param author - the author.
	 */
	public String getLongestBookWithGenre(String genre) {
		// TODO: implement
		return null;
	}
}