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()?

Answer 2

clear() will be much more efficient. It will simply remove each and every item. Using removeAll(arraylist) will take a lot more work because it will check every item in arraylist to see if it exists in arraylist before removing it.

Answer 3

They serve different purposes. clear() clears an instance of the class, removeAll() removes all the given objects and returns the state of the operation.