# Java: Collection Interface

The Collection interface is the parent of the List and Set interfaces, but not Map.

Assume the following declaration for identifiers in the table below:

    Collection coll; boolean b; Object obj; int i; Iterator it;

Returns | Method | Action
--- | --- | ---
Adding objects to a collection
`b =` | `coll.add(obj)` | Adds `obj` to this collection. Returns true if the collection was changed because of the add (eg, it will always be true for adding to a List, but will be false when adding to a Set that already contains this object).
`b =` | `coll.addAll(coll)` | Adds all elements of `obj` to this collection. Returns true if the collection was changed because of the addition.
Removing objects from a collection
`` | `coll.clear()` | Removes all elements from the collection.
`b =` | `coll.remove(obj)` | Removes one occurrence of `obj` from this collection. Returns true if the collection was changed because of this operation.
`b =` | `coll.removeAll(coll)` | Removes all elements of `coll` from this collection. Returns true if the collection was changed because of this operation.
`b =` | `coll.retainAll(coll)` | Removes all elements of which are not in `coll` from this collection. Returns true if the collection was changed because of this operation.
Testing for presence in a collection
`b =` | `coll.contains(obj)` | Returns true if `obj` is in this collection.
`b =` | `coll.containsAll(coll)` | Returns true if all the members of `coll` are in this collection.
`b =` | `coll.isEmpty()` | Returns true if there are no objects in this collection.
Other
`it =` | `coll.iterator()` | Returns an iterator for this collection. The order of elements is only specified if the underlying collection has an ordering (eg, List and TreeSet so, HashSet doesn't).
`i =` | `coll.size()` | Returns the number of objects in this collection.
`Object[] =` | `coll.toArray()` | Returns an array containing the elements of this collection.
`Object[] =` | `coll.toArray(Object[])` | Returns an array containing the elements of this collection.