1. Practical Session 4 Java Collections

2. Outline Working with a Collection The Collection interface
The Collection hierarchy
Maps
Case Study: Undoable Stack
The Collections class
Wrapper classes

3. Collection
A group of elements. The group size can be changed during run-time.
A collection object has many useful methods for manipulating the collection:
Inserting elements
Deleting elements
Copying the collection
Iterating over the elements
Computing subsets of elements …
Various types of collections are defined in the standard library:
Vector, ArrayList, Stack, ArrayDequeue, PriorityQueue, TreeSet, HashMap…

4. Collection Why shouldn’t we just use arrays? Arrays are not objects!
You cannot invoke methods on arrays.
Arrays have fixed size.
Arrays are less convenient for representing certain types of collections:
Sets
Double-ended queues
Dictionaries …

5. Working with a collection
Definition syntax: Collection <type> colName = new Collection <type>()
Example:
import java.util.Vector; …
Vector <Car> carsVec = new Vector <Car>();
Car Volvo = new Car(2.0);
Car BMW = new Car(2.2);
carsVec.add(Volvo);
carsVec.add(BMW);
for (Car c: carsVec)
System.out.println(c);

6. Working with a collection
Example - continued:
Car temp = carsVec.elementAt(0);
carsVec.set(0,carsVec.elementAt(1));
carsVec.set(1, temp);
for (int i=0; i < carsVec.size(); i++)
System.out.println(carsVec.elementAt(i));

7. The Collection Interface
Collection is an interface in the standard java library.
The Collection interface is generic. It accepts a type as a parameter.
public interface Collection<E> extends Iterable<E> {
int size();
boolean isEmpty();
boolean contains(Object element);
boolean add(E element); //optional
boolean remove(Object element); //optional
Iterator<E> iterator();
Object[] toArray();
<T> T[] toArray(T[] a); …

8. The Collection Hierarchy
(Partial illustration)
Collection
List
Set
Queue

9. The Collection Hierarchy
(Partial illustration)
Collection
List
Set
Queue
Vector

10. The Collection Hierarchy
(Partial illustration)
Collection
List
Set
Queue
Vector
ArrayList
PriorityQueue
Stack

11. Case Study: Undoable Stack
Various programs allow the user to undo his operations.
The undo operations are performed in reverse order.
In such a program, we need to add each operation with its parameters
onto the stack.

12. Program
Stack

13. Program
Stack
Resize 36

14. Program
Stack
Resize 36
Recolor 4
Resize 36

15. Program
Stack
Resize 24
Recolor 4
Resize 36
Undo:
Resize(36)

16. Program Stack Undo: Resize(36) Resize 24 Recolor 4 Resize 36 Recolor 4

17. Undoable Stack Hierarchy

18. Undoable Stack Hierarchy
TextArea
-text: String
-size: int
-color: int
+ getters…
+ setters…

19. Undoable Stack Hierarchy
TextArea
-text: String
-size: int
-color: int
+ getters…
+ setters…
Operation
+ perform(arg: int)
+ getArg() : int

20. Undoable Stack Hierarchy
TextArea
-text: String
-size: int
-color: int
+ getters…
+ setters…
Operation
+ perform(arg: int)
+ getArg() : int
Recolor
- color: int
+ perform(arg: int)
+ getArg() : int
Resize
- size: int
+ perform(arg: int)
+ getArg() : int

21. Undoable Stack Hierarchy
TextArea
-text: String
-size: int
-color: int
+ getters…
+ setters…
Operation
+ perform(arg: int)
+ getArg() : int
0..n
Recolor
- color: int
+ perform(arg: int)
+ getArg() : int
Resize
- size: int
+ perform(arg: int)
+ getArg() : int
UndoStack
+ add (op :Operation)
+ undo()

22. Maps An object that maps keys to values.
A map cannot contain duplicate keys.
each key can map to at most one value.

23. Maps 1 2 3 4 5 6 9
מערכים: 1 7 10 13 15 30 55 6 8 4 46 11
מפות:

24. Words count
Generates a frequency table of the words found in a sentence. Example: how much wood could a woodchuck chuck if a woodchuck could chuck wood. wood=2, could=2, how=1, if=1, chuck=2, a=2, woodchuck=2, much=1

25. Words count
String sentence = "how much wood could a woodchuck chuck if a woodchuck could chuck wood"; Map<String, Integer> map = new HashMap<String, Integer>(); for (String word : sentence.split(" ")) { Integer freq = map.get(word); map.put(word, (freq == null) ? 1 : freq + 1); }

26. Map types HashMap<K,V>.
This implementation provides constant-time performance for the basic operations (get and put).
Uses the hashCode() method.
String sentence = "how much wood could a woodchuck chuck if a woodchuck could chuck wood";
Map<String, Integer> map = new HashMap<String, Integer>();
{wood=2, could=2, how=1, if=1, chuck=2, a=2, woodchuck=2, much=1}

27. Map types TreeMap<K,V>.
The map is sorted according to the natural ordering of its keys,
or by a Comparator provided at map creation time.
This implementation provides guaranteed log(n) time cost for the containsKey, get, put and remove operations.
String sentence = "how much wood could a woodchuck chuck if a woodchuck could chuck wood";
Map<String, Integer> map = new TreeMap<String, Integer>();
{a=2, chuck=2, could=2, how=1, if=1, much=1, wood=2, woodchuck=2}

28. Map types LinkedHashMap<K,V>.
A HashMap that maintains a doubly-linked list.
This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).
String sentence = "how much wood could a woodchuck chuck if a woodchuck could chuck wood";
Map<String, Integer> map = new LinkedHashMap<String, Integer>();
{how=1, much=1, wood=2, could=2, a=2, woodchuck=2, chuck=2, if=1}

29. The Collections class
The Collections class contains static methods that operate on collections:
min
max
reverse
sort …
Example:
import java.util.Collections; import java.util.Vector; …
Vector <Car> carsVec = new Vector <Car>();
Collections.reverse(carsVec);

30. A wrapper class is an object representing a primitive type.
Wrapper Classes
What happens if we want to create a collection of a primitive type?
Collections can be made only of objects.
Primitive types are not objects.
We use wrapper classes instead.
A wrapper class is an object representing a primitive type.

31. Wrapper Classes Integer Float Double Character Boolean int float
Example:
Integer intObj = new Integer(10);
int number = intObj.intValue();

32. A collection of wrapper objects
Import java.util.Vector; …
Vector <Integer> numVec = new Vector <Integer>();
int size = 10;
for (int i = size; i >0; i--)
numVec.add(new Integer( i ));
Collections.sort(numVec);
for (Integer i: numVec)
System.out.println( i);