Why should the interface for a Java class be preferred?

Answer 1

In general for your line of code it does not make sense to bother with interfaces. But, if we are talking about APIs there is a really good reason. I got small class

class Counter {
    static int sizeOf(List items) {
        return items.size();
    }
}

In this case is usage of interface required. Because I want to count size of every possible implementation including my own custom. class MyList extends AbstractList....