Removing items from a list in Java

Question

In my computer science class, we use an online program called CodeHS that gives us assignments that cover certain topics. Usually, I understand ArrayLists relatively well, but this assignment uses Lists instead and it is kind of tripping me up. I have been struggling a little bit and my code (I will link it at the bottom) is returning the exact list that they insert in the method parameters. (in other words, my code is not doing what I want it to. It's not doing anything at all.) I am very new to this and would appreciate some help, with helpful criticism rather than scolding, haha. Thank you.

Here is the Assignment:

You’ve been given a list of books to read over the summer, but you need to trim down the list of books so you can finish all of them.

Write a method

public List<Book> filterBooks(List<Book> readingList, int maxPages)

That takes a List of Books as a parameter, removes all Books from the readingList that have more than maxPages pages, then returns the resulting list.

You can access the number of pages of a Book by calling book.getNumPages(). The Book class is provided for reference.

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

public Book(String theTitle, String theAuthor, int numberOfPages)
{
    title = theTitle;
    author = theAuthor;
    numPages = numberOfPages;
}

public String getTitle()
{
    return title;
}

public String getAuthor()
{
    return author;
}

public int getNumPages()
{
    return numPages;
}

public String toString()
{
    return title + " by " + author;
}

public boolean equals(Book other)
{
    return title.equals(other.title) && author.equals(other.author);
}
}

This is what I have tried:

public List<Book> filterBooks(List<Book> readingList, int maxPages)
{
Book currentBook;
ArrayList<String> readingList2 = new ArrayList<String>();
for(int i= 0; i < readingList.size(); i++)
{
    currentBook = readingList.get(i);
    if(currentBook.getNumPages() >= maxPages)
    {
       readingList.remove(currentBook);
    }
}

    return readingList;

}

Answer 1

Your code does not appear to be what you described. You should be removing some items from the original list, so it shouldn't return the exact same list, unless none of the books you checked matched the criteria for removal. (Assuming the list passed in is mutable.)

However you also have a couple bugs. Based on the problem description your condition should be > not >=. You also have a bug in the way you don't compensate for removing from the original list when you address the elements by index. If you remove an item at index i you should not increment i because the next book will now be in that position and you don't want to skip it. You will skip some books that you should have eliminated if there are two books in a row that should be removed.

Answer 2

You could use the List.removeIf(...) function, (if it's supported by the list) that does exactly what you want: remove all objects in that list if they meet a criteria.

So the code would be (with lambda):

readingList.removeIf(b -> b.getNumPages() <= maxPages);

Or without lambdas:

readingList.removeIf(new Predicate<Book>() {
    @Override
    public boolean test(Book b) {
        return b.getNumPages() <= maxPages;
    }
}