removeAll should not be used to clear a collectionJAVA-P1005

This code appears to clear a collection by passing a reference of the collection into its own removeAll() method.

This is very inefficient, as it is an operation with complexity O(n^2) (quadratic time) as opposed to a regular clear() call which is O(n) (linear time) complex.

When one calls a.removeAll(b) where a and b are Collections, we iterate over a, and check if b contains any element from a. If it does, we remove those elements. However, if we were to call removeAll() with a itself as its argument (like, a.removeAll(a)), we would iterate once over a for each element within a. This is a very inefficient operation.

Additionally, calling removeAll() in this way on thread-safe collections may throw a ConcurrentModificationException in some cases.

Bad Practice

someCollection.removeAll(someCollection);

Recommended

Just use clear() instead.

someCollection.clear();