Why parameter of contains() of list class differ from containAll()?

Question

Hi I checked Params of Contains methods and ContainsAll method .

boolean containsAll(Collection<?> c);

boolean contains(Object o);

It seems both will accept Object . Is there any difference among two.

I don't see what's unclear here? contains checks for one Object, and containsAll checks for one or more Objects. — Ceiling Gecko, Mar 20 '15 at 15:16
They don't both accept `Object`. The first one only accepts a `Collection`. — Paul Boddington, Mar 20 '15 at 15:18
_Returns true if this collection contains the specified element._ and _Returns true if this collection contains all of the elements in the specified collection._ I don't see how that can be any clearer. — Sotirios Delimanolis, Mar 20 '15 at 15:18
See Sytnax they could have written boolean containsAll(Collection — T-Bag, Mar 20 '15 at 15:18
If you pass a collection to both of them then `contains` will check if the actual collection is present, while `containsAll` will check if the elements within the collection are present. — Ceiling Gecko, Mar 20 '15 at 15:23
If you have a completely different question, ask it in a new question. — Sotirios Delimanolis, Mar 20 '15 at 15:27
Your real question: *what's the difference between `Collection>` and `Collection — Luiggi Mendoza, Mar 20 '15 at 15:40

score 2 · Answer 1 · answered Mar 20 '15 at 15:16

No, they do not both accept Object. Let us assume you have an Collection x:

The first method (x.containsAll(c)) accepts a Collection c and will return true if ALL elements in that Collection are also contained in x.

The second one (x.contains(o)) accepts an Object o and will return true if that Object is contained in x.

score 2 · Answer 2 · answered Mar 20 '15 at 15:23

The difference between contains and containsAll is that contains check if 1 Object (the parameter) exists in the list while containsAll check if the list contains ALL the elements in the given collection (hence the all in the method's name).

Also containsAll may accept Object because Object is the superclass of every class in Java. However, if the Object you pass is not an instance of Collection, you'll have a ClassCastException at runtime.

score 0 · Answer 3 · answered Mar 20 '15 at 15:17

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Yes, there is a difference. containsAll will not accept Object. It will only accept a Collections of Objects.

answered Mar 20 '15 at 15:17

Manu

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score 0 · Answer 4 · edited Mar 20 '15 at 15:36

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Method below accepts any type since all derive from Object

contains(Object o);

Method below accepts a collection of any type since wildcard ? is used:

containsAll(Collection<?> c);

edited Mar 20 '15 at 15:36

Luiggi Mendoza

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answered Mar 20 '15 at 15:28

S. Pauk

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read the comment on the second method, I mean that first accepts any type and the second accepts a collection of any type – S. Pauk Mar 20 '15 at 15:33
@LuiggiMendoza - updated so it reads more clear, still a downvote? – S. Pauk Mar 20 '15 at 15:34
I just removed the bad wording of the post. – Luiggi Mendoza Mar 20 '15 at 15:38
well, that was a crucial part of my answer since I wanted to stress that they have some 'synergy' – S. Pauk Mar 20 '15 at 15:39
And that's wrong. They don't have some synergy. They're different methods. – Luiggi Mendoza Mar 20 '15 at 15:40

Why parameter of contains() of list class differ from containAll()?

4 Answers4