1. Java Collections Framework
The client view

2. The Big Picture

3. Abstract Data Type Operations supported Defined as interfaces, such as
CRUD
Membership management
Misc.
Defined as interfaces, such as
Collection
List
Queue
Set
Iterable
ADT
data
CRUD

4. List: An ADT We Already Know
Real world example
Ways to manipulate
Shopping list
Elements listed in a certain order (position can be labelled with an index)
Add, remove, or modify anywhere in the list
Order in the list matters
Know what’s on the list or not
Know count of entries on the list
List<T> specs in Java API

5. List: The Abstract View
A Sequence of Elements
Two popular ways
With no rules how they can be accessed
To implement a list
Using an array: for fast access
Using a “link” structure: for easy insertion and deletion (in the middle or anywhere else) 1 2 3 4 5

6. Queue: Another Linear Structure
A Real World Example
Ways To Manipulate
A service queue
A line of customers to be serviced
General rule:
The head and tail are known
First come first serviced or FIFO
Size of the queue may be limited
Add: only to the back (or tail)  enqueue
Remove: only from the front (or head)  dequeue

7. Queue: The Abstract View
No arbitrary access
Java API
is allowed
Operations as specified in java.util.Queue<T> interface
dequeue
head
Throws exception
Returns special value
Insert
add(e)
offer(e)
Remove
remove()
poll()
Examine
element()
peek()
false
null
NoSuchElementException if queue is empty, or
IllegalStateException if size limit reached
tail
enqueue

8. Stack A real world example Ways To Manipulate
A pile of rings that can only be moved (on or off) from the top
LIFO: It’s opposite to the order enforced by queue
push
pop
top
Provided as a class since Java 1.0
Return Type
Method
boolean
empty() E
peek()
pop()
push(E item)
int
search(Object o)
Index is 1-based

9. Set: Unordered But No-duplicates
Mathematically, a set is defined as “A collection that contains no duplicate elements”, and ordering is not of concern.
Two implementing classes often used in Java API:
TreeSet<E>, in which elements are ordered using their natural ordering
HashSet<E>, for which no iteration ordering guaranteed

10. Map: Collection of K-V Pairs
In the Java API, Map is specified as an interface:
An object that maps keys to values.
A map cannot contain duplicate keys;
each key can map to at most one value.
Map<K,V>
Map.Entry
The CRUD Operations C 
put(K key, V value) : insert a new entry when the key is not in the Map yet R
get(Object key) U
put(K key, V value): replace the value associate with an existing key to a new value D
remove(Object key)
Key set
Value
collection

11. Iterable: Super-Interface of All
All Collection objects need to provide a way to guide its client to iterate (or visit each element just once) through its collection.
Made available by implementing the iterator() method as specified in the Collection interface since Java 1.2.
Iterable, a super interface to Collection is added in Java 1.5:
This interface requires only one method: iterator().
Implementing this interface allows an object to be the target of the "for-each" statement.
Iterable<T> c = …;
for (T t : c)
System.out.println(t);
Iterable<T> c = …;
Iterator<T> itr = c.iterator();
while(itr.hasNext())
System.out.println(itr.next());
Iterator used implicitly.

12. <<interface>>
Iterating thru a Map
Map is not a subtype of Iterable. Its contents need to be iterated based on
Its key set, or
Its value collection
<<interface>>
Iterable
values() returns all values in a Collection
keySet() returns all keys in a Set

13. Map as a Frequency Table
Map<Character,Integer> freqTbl = TreeMap();
String text = …;
Character ch;
for (int i=0; i<text.length(); i++) {
ch = text.charAt(i);
if (freqTbl.get(ch) == null) freqTbl.put(ch, 1);
else
freqTbl.put(ch, freqTbl.get(ch) + 1); }
Set<Character> chs = freqTbl.keySet();
for (Character c : chs)
System.out.println(c + " --> " freqTbl.get(c));

14. A Map Of Lists
In the Jeopardy! game, a map can be used to relate category names to the lists of questions
Key
Map<K,V> qByCats = new Map<K,V>();
Set<K> cats = qByCats.keySet();
for (K key : cats)
System.out.println(key + " --> " qByCats.get(key));
Value
A --> [A0, A1, A2, A3, A4]
B --> [B0, B1, B2, B3, B4]
C --> [C0, C1, C2, C3, C4]
D --> [D0, D1, D2, D3, D4]
E --> [E0, E1, E2, E3, E4]
F --> [F0, F1, F2, F3, F4]

15. A Map Of Lists Display the map contents in the desired way Key Value
Map<K,V> qByCats = new Map<K,V>();
Set<K> cats = qByCats.keySet();
//display category names in a row
for (String k : keys) {
System.out.print(k + '\t'); }
System.out.println();
//list as a column beneath category
for (int i=0; i<5; i++) {
System.out.print(m.get(k).get(i) '\t');
Key
Value
A B C D E F
A0 B0 C0 D0 E0 F0
A1 B1 C1 D1 E1 F1
A2 B2 C2 D2 E2 F2
A3 B3 C3 D3 E3 F3
A4 B4 C4 D4 E4 F4

16. Sorting w/ a New Order import java.util.Comparator;
for (String k : keys) {
System.out.print(k + ‘\t');
Collections.sort(m.get(k), c); }
Comparator<String> c = new Comparator<String>() {
@Override
public int compare(String priceS1, String priceS2) {
return Integer.parseInt(priceS1.substring(1))
- Integer.parseInt(priceS2.substring(1)); } };
Sorting w/ a Comparator
Format to a fixed length
System.out.print(
formatToLength(colWidth, k)
+ ' ');