Java Reference
In-Depth Information
Self-Test Exercises
8. Does a
HashSet<T>
object have a method to produce a
ListIterator<T>
? Does an
ArrayList<T>
object have a method to produce a
ListIterator<T>
?
9. Suppose
i
is a
ListIterator<T>
. Will an invocation
of i.next( )
followed by
i.previous( )
return the same element for each of the two invocations or might they
return two different elements? What about
i.previous( )
followed by
i.next( )
?
Chapter Summary
•
The main collection interfaces are
Collection<T>
,
Set<T>
, and
List<T>
. The
Set<T>
and
List<T>
interfaces extend the
Collection<T>
interface. The library
classes that are standard to use and that implement these interfaces are
Hash-
Set
<
T
>, which implements the
Set
<
T
> interface, and
ArrayList<T>
, which
implements the
List<T>
interface.
•
A
Set<T>
does not allow repeated elements and does not order its elements. A
List<T>
allows repeated elements and orders its elements.
•
The
Map<K,V>
interface is used to store a mapping between a key
K
and a value
V
.
It is commonly used to store databases in memory. The
HashMap<K,V>
class is a
standard library class that implements a map.
•
An iterator is something that allows you to examine and possibly modify the ele-
ments in a collection in some sequential order. Java formalizes this concept with
the two interfaces
Iterator<T>
and
ListIterator<T>
.
•
An
Iterator<T>
(with only the required methods implemented) goes through the
elements of the collection in only one direction, from the beginning to the end. A
ListIterator<T>
can move through the collection list in both directions, forward
and back. A
ListIterator<T>
has a set method; the
Iterator<T>
interface does
not require a set method.
Answers to Self-Test Exercises
1.
public
static
<T>
boolean
inSome(T target,
Collection<T> c1, Collection<T> c2)
{
return
(c1.contains(target) || c2.contains(target));
}
2.
public
static
<T> T getFirst(List<T> aList)
{
if
(aList.isEmpty( ))
return
null
;
else
