Why does some libs define their own collections?

The Elements class in JSoup has many specific methods to its function e.g.

toggleClass(String className) 
html()

Which wouldn't be available on ArrayList. I guess ArrayList could have been extended, but the author of Elements has gone for composition. Elements HAS-A ArrayList instance which it keeps private. Elements exposes methods of the List interface, bespoke methods and others.

But Elements is backed by an ArrayList - this way the author can also decorate the methods of ArrayList he chooses to make public via his own Class. This is an extract from the source code:

  private List<Element> contents;

    public Elements() {
        contents = new ArrayList<Element>();
    }

BTW: You used to see wrappers pre Java 5 to give type safety to Java Collections, but since Java 5 Generics have stopped a lot of this.