1. Some Collections: BAGS, SETS, and STACKS
COMP 103
Some Collections: BAGS, SETS, and STACKS

2. RECAP-TODAY 2
RECAP
Type Parameters (any Object type, ‘wrapped’ primitive types)
Autoboxing
ArrayList and List example
TODAY
Iterators
More Collections: Bags, Sets, Stacks

3. You have already met one: scanner is a (rather fancy) iterator!
Iterators
List <String> items;
for ( String str : items) UI.print(str + “, “);
How does this work?
The compiler essentially turns it into this:
Iterator<String> iter = items.iterator( );
while (iter.hasNext( )){
String str = iter.next( );
UI.print(str + “, “); } Do
Foe
Woe
Zoo Go
Row So
hasNext()
next()
An iterator object attached to items that will keep giving you the next element
You have already met one: scanner is a (rather fancy) iterator!

4. Iterator is an Interface…4
Set <Task> mySet = new ArraySet <Task> ();
Iterator <Task> iter = mySet.iterator();
mySet
iter
Conforms to the interface Iterator
so… it has methods
hasNext()
next()
remove()
Conforms to the
interface Set
so… has methods
add()
remove()
contains(), etc…

5. Collections library Interfaces: Classes: = Bag (most general)5
Interfaces:
Collection <E >
= Bag (most general)
List <E >
= ordered collection
Set <E >
= unordered, no duplicates
[Stack <E > (not an interface!)
ordered collection, limited access
(add/remove at one end) ]
Map <K, V >
= key-value pairs (or mapping)
Queue <E >
(add at end, remove from front)
Classes:
List classes:
ArrayList, LinkedList, Vector
Set classes:
HashSet, TreeSet, EnumSet, LinkedHashSet,…
Stack classes:
Stack
Map classes:
EnumMap, HashMap, TreeMap, LinkedHashMap, WeakHashMap, … …

6. Bags (“Collection”) A Bag is a collection with Minimal Operations:6
A Bag is a collection with
no structure or order maintained
no access constraints (access any item any time)
duplicates allowed
Minimal Operations:
add(value) → returns true iff a collection was changed
remove(value) → returns true iff a collection was changed
contains(value) → returns true iff value is in bag uses equal to test.
findElement(value) → returns a matching item, iff in bag
Plus
size(), isEmpty(), iterator(), clear(), addAll(collection), removeAll(collection)…

7. Bag Applications When to use a Bag?7
When to use a Bag?
When there is no need to order a collection, and duplicates are possible:
A collection of current logged-on users.
Bingo cards.
The books in a book collection …
But not all that common! Typically we don’t really have duplicate items.
There are no standard implementations of Bag!!

8. Sets Set is a collection with:8
Set is a collection with:
no structure or order maintained
no access constraints (access any item any time)
Only property is that duplicates are excluded
Operations: (same as Bag, but different behaviour)
add(value) → true iff value was added (eg, not duplicate)
remove(value) → true iff value removed (was in set)
contains(value) → true iff value is in the set
findElement(value) → matching item, iff in value in the set …
Sets are as common as Lists

9. check if a particular word is in the set
Using Sets 9
Checking vocabulary of children’s books:
private Set <String> words = new HashSet <String> ();
public void readBook(Scanner sc){
while (sc.hasNext ()) words.add(sc.next()); }
public void checkWord(String wd){
if (words.contains(wd))
UI.println(“Yes, ”+wd+“ is in the book”);
else
UI.println(“No, ”+wd+“ is not in the book”);
make an empty set
add words to it
check if a particular word is in the set

10. Stacks Works on the “first in last out” (FILO) principle10
Works on the “first in last out” (FILO) principle
Stacks are a special kind of List:
Sequence of values,
Constrained access: add, get, and remove only from one end.
Java should have a Stack interface, and different implementations of it (ArrayStack, LinkedStack, etc), but...
In Java Collections library:
Stack is a class that implements List (actually extends Vector)
Has extra operations: push(value), pop(), peek()

11. Stacks Stacks have extra operations: push(value), pop(), peek()11
Stacks have extra operations: push(value), pop(), peek()
pop(): Removes and returns top of stack
push(value): Put value on top of stack
peek(): Returns top of stack, without removing
plus the other List operations (oops!)

12. Stacks example Reversing the items from a file:12
Reversing the items from a file:
read and push onto a stack
pop them off the stack
public void reverseNums(Scanner sc){
Stack <Integer> myNums = new Stack <Integer> ();
while (sc.hasNextInt())
myNums.push(sc.nextInt())
while (! myNums.isEmpty())
UI.print(myNums.pop() + “\n”); }

13. Applications of Stacks13
Programs that deal with programs
eg, program execution,
eg, expression execution,
(6 + 4) * ((10 * √64) – (√81/ 3))
working on subtasks, then returning to previous task.
Processing files of structured (nested) data.
eg. reading files of with structured markup (HTML, XML,…)
Undo in editors

14. Stack for evaluating expressions14
(6 + 4) * ((10 * √64) – (√81/ 3))
⇒ ( (6, 4)+ , ((10, (64)√) * , ((81)√, 3)/ )- )*
⇒ √ * 81 √ 3 / *
if number: push on stack
if operator: pop arguments from stack compute value push on stack
if done: pop answer from stack
Postfix order
(also known as “Reverse Polish notation”

15. Summary 15
Iterators
Bags
Sets
Stacks