public abstract boolean contains (Object o)

Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).

Parameters:
o    element whose presence in this collection is to be tested

Returns:  true if this collection contains the specified element

Exceptions:
ClassCastException    if the type of the specified element is incompatible with this collection (optional)
NullPointerException    if the specified element is null and this collection does not permit null elements (optional)