Collections - The Java Programming Language - page 849

21.4. The Collection Interface
As you have seen, most collection types are subtypes of the Collection
interface. The only exceptions are those that are subtypes of Map . In
this section we cover the Collection interface. The following sections de-
scribe the interfaces that extend Collection and the specific collection im-
plementations provided in java.util . We leave Map and its related types
for later. We also defer talking about implementing your own collection
types.
The basis of much of the collection system is the Collection interface. As
you saw in Figure 21-1 , most of the actual collection types implement
this interface, usually by implementing an extended interface such as Set
or List . So Collection is a good place to start understanding collections.
It has the following primary methods for working with an individual col-
lection:
public int size()
Returns the size of this collection, that is, the number of ele-
ments it currently holds. The value returned is limited to In-
teger.MAX_VALUE even if the collection holds more elements.
public boolean isEmpty()
Returns TRue if this collection currently holds no elements.
public boolean contains(Object elem)
Returns true if the collection contains the object elem ; that is,
if this collection has an element on which invoking equals with
elem returns true . If elem is null , returns TRue if there is a null
element in the collection.
public Iterator<E> iterator()