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CMSC 433 – Programming Language
Technologies and Paradigms
Spring 2007
Inner Classes & Iterators
Mar. 6, 2007
Inner Classes
Classes can be nested inside other classes
– These are called
inner classes
use the inner class just like any other classWithin a class that contains an inner class, you can
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Example: The Queue Class
class
Queue {
class Entry
Java
inner class
Element elt; Entry
next;
Entry(Element
i) {
elt
= i;
next =
null; }
void enqueue(ElementEntry theQueue;}
e) {
if (theQueue ==
null) theQueue
= new Entry(e);
else
Entry last =
theQueue;
while (last.next
!= null)
last =
last.next;
last.next =
new Entry(e);
Example: The Queue Class (cont’d)
class
Queue {
Element dequeue() throws...
EmptyQueueException
if (theQueue
== null)
throw
new EmptyQueueException();
Element
e = theQueue.elt;
theQueue
= theQueue.next;
return e;
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Referring to Outer Class
class Queue
class Entry {int numEntries;...
Entry(Element i) { elt = i; nextElement elt; Entry next;
= null;
numEntries++;
the outer “object” whose method created itEach inner “object” has an implicit reference to
– Can refer to fields directly, or use outer class name.
Anonymous Inner Classes
(new
Thread() {
public void run() {
} System.exit(1);System.out.println(”...");Thread.sleep(10006020);try {
catch
(Exception e)
}).start();
method on itCreate anonymous subclass of thread, and invoke
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Other Features of Inner Classes
to refer to type of inner classOutside of the outer class, use outer.inner notation
– E.g., Queue.Entry
An inner class marked
static
does not have a
reference to outer class
– Must also use outer.inner notation to refer to inner class– Can’t refer to instance variables of outer class
Compiling Inner Classes
The JVM doesn’t know about inner classes
– Anonymous inner class of outer class A produces– Inner class Foo of outer class A produces A$Foo.class– Compiled away, similar to generics
A$1.class
Why are inner classes useful?
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Using Iterators (contd…)
private static
void
useWhileLoop(
Collection
aFlavours
Iterator flavoursIter
= aFlavours.iterator();
while
( flavoursIter.hasNext()
System.out.println( flavoursIter.next() );
Note
that
this
for-loop
does
not use an
integer
index.
private*/
static void useForLoop(
Collection
aFlavours
for (
Iterator
flavoursIter =
aFlavours.iterator();
flavoursIter.hasNext();
System.out.println( flavoursIter.next() );
Iterators and Queues
Recall queue example from beginning of lecture
We’ll explore options for adding iterators
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next() Shouldn’t Mutate Aggregate
class
Queue {
class QueueIterator implements...
Iterator {
Entry
rest;
QueueIterator(Entry
q)
{ rest
= q;
boolean hasNext() {
return
rest !=
null; }
Element next()
throws NoSuchElementException
if
(rest
null)
throw new NoSuchElementException();
Element
e =
rest.elt;
rest =
rest.next;
queue
data
intact
return
e;
Evil Mutating Clients
But a client could mutate the data structure …
HashMap h
= ...;
... //entrySet() Returns a
collection view of
the mappings
//contained in
this map.
Iterator i
= h.entrySet().iterator();
System.out.println(i.next());System.out.println(i.next()); //// put(Object key, Object value)
Associates the
specified value
//with the specified key in this map.
h.put(“Foo”, “Bar”); //
hash table resize!
System.out.println(i.next());
//
prints ???
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Defensive (Proactive) Copying
Solution 1: Iterator copies data structure
class
QueueIterator
implements
Iterator
Entry
rest;
QueueIterator(Queue
q)
{
copy
q.theQueue to
rest
Pro: Works even if queue is mutated
Con: Expensive to construct iterator
Timestamps
Solution 2: Track Mutations
class
Queue {
int...
modCount
void enqueue(Element
e) {
modCount++;
Element
dequeue()
{ ... modCount++;
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Timestamps (cont’d)
class QueueIterator implements ...
Iterator {
int expectedModCount =
modCount; // set
at
iterator
construction time
Element next()
if
(expectedModCount
!= modCount)
throw
new
ConcurrentModificationException();
// does hasNext() need}
to be
modified?
Pro: Iteration construction cheap
Con: Doesn’t allow any mutation
What if Mutation is Allowed?
Allowed mutation must be part of iterator spec
public
void
remove()
throws IllegalStateException;
per call to next.the iterator (optional operation). This method can be called only onceRemoves from the underlying collection the last element returned by
calling this method.modified while the iteration is in progress in any way other than byThe behavior of an iterator is unspecified if the underlying collection is
