Java Reference
In-Depth Information
Note
The order of the lines in the output may be different when you run the
StockPortfolio
class in your environment because the underlying implementation
uses a
HashMap
. A
HashMap
does not guarantee the order of the elements stored in
the map, and this extends to its iterators. If you wanted the iterator to return elements
sorted by the stock symbol, you could use one of the sorted collections, such as
TreeMap
or
TreeSet
, instead of
HashMap
. Another option is to utilize a stream on
the collection, which is a feature that was introduced in Java 8. See Recipe 7-8 for more
about streams.
How It Works
The
Iterable
interface was introduced in Java 5 to support the enhanced
for
loop
(also known as the
foreach
loop) which was introduced at the same time. Along
with these enhancements to the language, all
Collection
classes were retrofitted to
implement the
Iterable
interface, thus allowing
Collection
classes to be iter-
able using the
foreach
loop. The
Iterable
interface is a generic type defined as
follows:
public interface Iterable<T> {
Iterator<T> iterator();
}
Any class that implements
Iterable<T>
must implement the
iterator()
method to return an
Iterator<T>
object. Typically, the
Iterator
returned is the
default iterator of the underlying collection; however, it may also return an instance of
a custom
Iterator
. In the
StockPortfolio
class, a
Map
is used to represent the-
stock portfolio. The key for each map entry is the stock symbol, and the value associ-
ated with each key is a
Stock
object.
Maps
in Java are not iterable; that is, they are
not
Collection
classes. Therefore, they do not implement
Iterable
. However,
both the keys and the values of a map are
Collections
, and therefore are
Iter-
ables
. We want our implementation of the
Iterable iterator()
method to re-
turn an
Iterator
over the values (
Stock
references) of the portfolio map; there-
fore, our
Iterable
implementation is parameterized by the
Stock
type:
public class StockPortfolio implements Iterable<Stock>
