Java Reference
In-Depth Information
Iterator and the Enhanced For Statement
Byextending
Iterable
,
Collection
inheritsthatinterface's
iterator()
meth-
od, which makes it possible to iterate over a collection.
iterator()
returns an in-
stance of a class that implements the
Iterator
interface, whose generic type is ex-
pressed as
Iterator<E>
and which declares the following three methods:
•
boolean hasNext()
returnstruewhenthis
Iterator
instancehasmore
elements to return; otherwise, this method returns false.
•
E next()
returns the next element from the collection associated with
this
Iterator
instance, or throws
java.util.NoSuchElementException
when there are no more ele-
ments to return.
•
void remove()
removes the last element returned by
next()
from the
collectionassociatedwiththis
Iterator
instance.Thismethodcanbecalled
onlyonceper
next()
call.Thebehaviorofan
Iterator
instanceisunspe-
cifiedwhentheunderlyingcollectionismodifiedwhileiterationisinprogress
inanywayotherthanbycalling
remove()
.Thismethodthrows
Unsuppor-
tedOperationException
when it is not supported by this
Iterator
,
and
IllegalStateException
when
remove()
hasbeencalledwithout
a previous call to
next()
or when multiple
remove()
calls occur with no
intervening
next()
calls.
The following example shows you how to iterate over a collection after calling
iterator()
to return an
Iterator
instance:
Collection<String> col = ... // This code does not compile
because of the “...”.
// Add elements to col.
Iterator iter = col.iterator();
while (iter.hasNext())
System.out.println(iter.next());
The while loop repeatedly calls the iterator's
hasNext()
method to determine
whetherornotiterationshouldcontinue,and(ifitshouldcontinue)the
next()
method
to return the next element from the associated collection.
Because this idiom is commonly used, Java 5 introduced syntactic sugar to the for
statement to simplify iteration in terms of the idiom. This sugar makes this statement
appearliketheforeachstatementfoundinlanguagessuchasPerl,andisrevealedinthe
following simplified equivalent of the previous example:
