Solutions to last two problems

Solutions.java

/* Returns all books in this library that are
 * written by the given author.
 *
 * @param author - the author.
 */
public ArrayList<Book> getBooksByAuthor(String author) {
	// Two comments:
	//
	// (1) Your solution is pretty close but it returns
	//     a SINGLE book by the given author, not all
	//     the books by the given author.
	//
	// (2) Note that the return type has to be ArrayList<Book>
	//     because the method definition is
	//     public ArrayList<Book> getBooksByAuthor
	ArrayList<Book> booksByAuthor = new ArrayList<Book>();

	// Then we find the the books with the same author...
	for (int i = 0; i < books.size(); i++) {
		Book book = books.get(i);
		if (book.getAuthor().equals(author)) {
			//...and add each valid book to our ArrayList of 
			// books by that author.
			booksByAuthor.add(book);
		}
	}

	// And then we return the whole ArrayList - so we return
	// all books by that author.
	return booksByAuthor;
}

/* Returns the title of the longest book (in pages) 
 * with the given genre.
 *
 * @param author - the author.
 */
public String getLongestBookWithGenre(String genre) {
	// A few comments:
	// 
	// This was probably the trickiest one..you were kinda close. :)
	//
	// (1) In this case, you can't set the longest book variable to
	//     books.get(0). This is because the Book at books.get(0)
	//     might not have the correct genre. This is where "null"
	//     is a good idea because it's a placeholder value that
	//     stands for the fact you don't have anything yet.
	//
	// (2) Also books.get(0) gives you a Book, not a String,
	//     since books is an ArrayList of Books. So the variable
	//     type is Book, not String.
	Book longest = null;

	// Then you have to loop through all the books
	for (int i = 0; i < books.size(); i++) {
		Book book = books.get(i);

		// (3) Now we have to check a few things here. A book is a 
		// valid candidate for the longest book IF it has the
		// correct genre AND it has more pages than our previous
		// candidate for the longest book.
		//
		// Alternatively, if we DON'T currently have a candidate
		// for the longest book (remember 'longest' starts out with
		// the placeholder value of 'null'), then we simply have to
		// check that the book has the right genre.
		//
		if (book.getGenre().equals(genre) &&
			(longest == null || book.getNumberOfPages() > longest.getNumberOfPages())) 
		{
			longest = book;
		}

		// (4) Beware of an early return statement!
		// The return statement needs to go outside the
		// for loop because you need to look at every single book.
		// in order to find which one is the longest.
		//
		// In the other questions I asked, it was okay to have a
		// return statement inside the for-loop because you only
		// had to find a single book that met the critera (like
		// a book that had a given author or a given title). 
		// Once you found that book, you didn't have to keep checking 
		// the rest of the books.
		//
		// However, if the criteria is something like the "longest"
		// book (out of all the books), then you obviously have to check
		// every single book. So the return statement must go outside.
	}

	if (longest != null) {
		return longest.getTitle();
	}
	else {
		// (5) Maybe none of the books in the library had the correct
		// genre. In thise case, 'longest' is still 'null'.
		// Since we didn't find anything, a good value to return
		// is null, which is a placeholder value that basically
		// means blank aka nothing aka...uhh you get the idea.
		return null;
	}
}