Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.)
Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.
If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns.
|element whose presence in this collection is to be ensured|
|if the add operation is not supported by this collection|
|if the class of the specified element prevents it from being added to this collection|
|if the specified element is null and this collection does not permit null elements|
|if some property of the element prevents it from being added to this collection|
|if the element cannot be added at this time due to insertion restrictions|