Collection<? extends T> vs Collection<T>

Question

After trying to understand the concepts at Spring MVC, I came across the expression Collection<? extends Book> which I have never seen before. I have tried to figure it out on my own, but I am seeing no difference between using Collection<? extends Book> and Collection<Book>. I was guessing that it only allowed for extensions of Book, but it does allow for Book as well. So scratch that. I have tried using Google, but since ? is a wildcard in google, it makes it nearly impossible to search for. I have searched stackoverflow for the answer, but all questions about this (such as List<? extends MyType> and <? extends > Java syntax) already assume knowledge of Collection<? extends T>. Here is the code that has initially intrigued my interest:

import java.util.ArrayList;
import java.util.Collection;

public class Book {
    public static void main(String[] args) {
        BookCase bookCase1 = new BookCase();
        BookCase bookCase2 = new BookCase(bookCase1);
    }
}

class BookCase extends ArrayList<Book> {
    public BookCase() {
    }

    //acts same as public BookCase(Collection<Book> c) {
    public BookCase(Collection<? extends Book> c) {
        super(c);
    }
}

What does <? extends T> do? How does it differ from <T>?

EDIT:

Followup question: Does BookCase extends ArrayList<Book> mean that BookCase extends Book?

Answer 1

Consider the following

class Animal { }
class Horse extends Animal { }

private static void specific(List<Animal> param) { }
private static void wildcard(List<? extends Animal> param) { }

Without the extends syntax you can only use the exact class in the signature

    specific(new ArrayList<Horse>()); // <== compiler error

With the wildcard extends you can allow any subclasses of Animal

    wildcard(new ArrayList<Horse>());  // <== OK

It's generally better to use the ? extends syntax as it makes your code more reusable and future-proof.