使用一个对象设置从集合中删除元素的优点

Question

I found next code in official java docs :

collection.removeAll(Collections.singleton(element));

I couldn't figure out advantages of this approach. Why not usual element removing?

collection.remove(element);

Thanks!

Answer 1

前者删除集合中所有出现的元素，后者仅删除第一次出现的元素。

Answer 2

Acc. to docs:

consider the following idiom to remove all instances of a specified element, e, from a Collection, c

collection.removeAll(Collections.singleton(element));

while collection.remove(element); removes a single instance of the specified element from this collection

So, to remove all instances, you've to use a loop construct with latter approach. while with first one it's just one line job.

Answer 3

From the API

remove(Object o)
Removes a single instance of the specified element from this collection, if it is present

and:

removeAll(Collection c)
Removes all of this collection's elements that are also contained in the specified collection

So, if your collection has more than one occurance of the element you wish to remove, you would have to execute collection.remove(element) several times, but removeAll only once. As removeAll takes a collection as argument, Collection.singleton can offer you a convenient way to create a collection with only one argument.

Answer 4

When testing(unit testing). Collections.singleton is a one line way to turn your subject into a collection. The alternative is two lines and not onerous.