What is the difference between ArrayList.clear() and ArrayList.removeAll()?

Question

Assuming that arraylist is defined as ArrayList arraylist, is arraylist.removeAll(arraylist) equivalent to arraylist.clear()?

Answer 1

Unless there is a specific optimization that checks if the argument passed to removeAll() is the collection itself (and I highly doubt that such an optimization is there) it will be significantly slower than a simple .clear().

Apart from that (and at least equally important): arraylist.removeAll(arraylist) is just obtuse, confusing code. It is a very backwards way of saying "clear this collection". What advantage would it have over the very understandable arraylist.clear()?