1. Software Development Iterators
Computer Science 209
Software Development
Iterators

2. Implementing equals for Sets
public boolean equals(Object other){
if (this == other) return true;
if (! (other instanceof Set)) return false;
List<E> otherSet = (Set)other;
if (this.size() != otherSet.size()) return false;
return this.containsAll(otherSet); }
Works correctly, because order of items is not implied
Worst-case quadratic running time

3. Implementing equals for Lists
public boolean equals(Object other){
if (this == other) return true;
if (! (other instanceof List)) return false;
List<E> otherList = (List)other;
if (this.size() != otherList.size()) return false;
for (int i = 0; i < this.size(); i++)
if (! this.get(i).equals(otherList.get(i)))
return false;
return true; }
Works correctly by accounting for the order of the items
get runs in constant time for ArrayList but in linear time for LinkedList
Worst-case quadratic running time for LinkedList.equals

4. Implementing equals with iterator
public boolean equals(Object other){
if (this == other) return true;
if (! (other instanceof List)) return false;
List<E> otherList = (List)other;
if (this.size() != otherList.size()) return false;
Iterator<E> otherIter = otherList.iterator();
for (E thisElement : this)
if (! thisElement.equals(otherIter.next()))
return false;
return true; }
Works correctly by accounting for the order of the items
next runs in constant time for any collection’s iterator
Worst-case linear running time for equals with lists

5. The Iterator Interface
public interface Iterator<E>{
public boolean hasNext()
public E next()
public void remove() }
remove deletes the item most recently accessed with next
remove must be included in the implementing class, but need not be supported (can throw an UnsupportedOperationException)

6. Using an Iterator
// add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
Every collection class that supports an iterator must provide an
iterator method.
This method returns an instance of a class that implements the Iterator interface
A sequence of elements
anIterator
aCollection

7. Using an Iterator collection D D D iterator
// add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
while (iter.hasNext()){
SomeType obj = iter.next();
System.out.println(obj); }
collection D D D
iterator

8. Using an Iterator collection D D D iterator
// add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
while (iter.hasNext()){
SomeType obj = iter.next();
System.out.println(obj); }
collection D D D
iterator

9. Using an Iterator collection D D D iterator
// add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
while (iter.hasNext()){
SomeType obj = iter.next();
System.out.println(obj); }
collection D D D
iterator

10. Using an Iterator collection D D D iterator
// add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
while (iter.hasNext()){
SomeType obj = iter.next();
System.out.println(obj); }
collection D D D
iterator

11. Iterable and the for-each Loop
// add a bunch of objects to col
for (SomeType obj : col)
System.out.println(obj);
If col implements the Iterable interface, the client can use a
for-each loop instead
collection D D D
iterator

12. Use in AbstractCollection
abstract public class AbstractCollection<E> implements
Collection<E>{
public void clear(){
Iterator<E> iter = this.iterator();
while (iter.hasNext()){
iter.next();
iter.remove(); }
public boolean remove(Object o){
while (iter.hasNext())
if (iter.next().equals(o)){
return true;
return false;

13. Preconditions on Methods
public boolean hasNext()
public E next()
hasNext has no preconditions
next has two preconditions:
hasNext returns true
the underlying collection has not been modified by one of
that collection’s mutators during the lifetime of that
iterator

14. Error: Run Out of Elements
// Add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
while (iter.hasNext()){
SomeType obj = iter.next();
<blah blah blah> }
SomeType obj = iter.next(); // This should cause an exception

15. Error: Inconsistent Data
// Add a bunch of objects to col
Iterator<SomeType> iter = col.iterator();
while (iter.hasNext()){
col.add(someItem);
SomeType obj = iter.next(); // This should cause an exception }
Using mutators in conjunction with iterators is a bad practice

16. An Iterator Implementation
public interface TrueStack<E> extends Collection<E>{
public E pop();
public void push(E newElement);
public E peek(); }
<<Interface>>
Iterable
<<Interface>>
Collection
<<Interface>>
TrueStack
The iterator method is in the Iterable interface

17. A Naïve Iterator Implementation
public class ArrayStack<E> extends AbstractCollection<E>
implements TrueStack<E>{
private List<E> list;
// Code for constructors, push, pop, peek, and size
public Iterator<E> iterator(){
return list.iterator(); }
Problem: a list’s iterator supports the remove method
This would violate the spirit of a stack

18. Another Design Strategy
By using the list’s iterator, we also expose the list to the client (violates the Law of Demeter)
Let’s continue to use it, but wrap our own iterator object around it
That allows us to control what’s supported and what’s not

19. An Iterator Implementation
public class ArrayStack<E> extends AbstractCollection<E>
implements TrueStack<E>{
private List<E> list;
// Code for push, pop, peek, and size
// Code for the iterator method
// Code for the class that implements the Iterator
// interface }
Define the iterator class as a private inner class.

20. The Implementing Class
public Iterator<E> iterator(){
return new StackIterator<E>(this.iterator()); }
private class StackIterator<E> implements Iterator<E>{
public boolean hasNext(){
return false;
public E next(){
return null;
public void remove(){
Nested
within
ArrayStack

21. The Implementing Class
public Iterator<E> iterator(){
return new StackIterator<E>(list.iterator()); }
private class StackIterator<E> implements Iterator<E>{
private Iterator<E> iter;
private StackIterator(Iterator<E> iter){
this.iter = iter;
// Other methods
Nested
within
ArrayStack
The stack iterator encapsulates the list iterator, hiding it from the client (proxy pattern)

22. Other Methods
private class StackIterator<E> implements Iterator<E>{
private Iterator<E> iter;
public boolean hasNext(){
return iter.hasNext(); }
public E next(){
return iter.next();
public void remove(){
throw new UnsupportedOperationException(
"remove not supported by stacks");
Must add javadoc with preconditions and code to check them

23. For Wednesday
GUIs with Swing and AWT