Java Collection Framework

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Collections Framework

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Collections Framework
A Map is not a true Collection.

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Collections Framework
A collections framework is a unified architecture
for representing and manipulating collections.
It consists of
o Interfaces – Collection, Set, List, Queue
o Concrete Classes – ArrayList, Hashmap, HashSet,
etc.
o Algorithm - searching and sorting

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Example
SET [1,2,4,5,6,7,8]
LIST [1,2,4,5,7,3,3,4,5]
QUEUE [1,2,4,5,7,3,3,4,5]
MAP [ {1,”ONE”}, {2,”TWO”}, {3,”THREE”} ,
{4,”THREE”}]

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Collection Interfaces
 Collection-A collection represents a group of objects known as
its elements.
 Set - cannot contain duplicate elements.
 List- ordered collection, can contain duplicate elements.
 Queue - holds multiple elements prior to processing. a Queue
provides additional insertion, extraction, and inspection
operations.
 Map - an object that maps keys to values. A Map cannot contain
duplicate keys.
 SortedSet - a Set that maintains its elements in ascending
order.
 SortedMap - a Map that maintains its mappings in ascending
key order.

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List by regular array
public static void main(String[] args) {
o String[] names = new String[5];
o for(int i=0;i<names.length; i++){
• names[i]= " No # " + i;
o }
o System.out.println("Names =" + names.length);
}
Disadvantages:
o All elements should be of same type, here it is String.
o Primitive data type arrays can be defined.
o Array size is Fixed. Defined at the time of creation.

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List by Collection framework
 public static void main(String[] args) {
o ArrayList names = new ArrayList();
o for(int i=0;i<10; i++){
• names.add(" No # " + i);
o }
o System.out.println("Names =" + names.size());
o Object o = names.get(0);
o String name = (String) o;
o //String name = (String) names.get(0);
o System.out.println(“ First Name is " + name);
 }
 Advantages:
o Unlimited size,no need to define array size at the time of creation.
o Add object of any class, in other words any object in Java.
o Add only objects but NOT primitive data types.

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Interface Collection
add(o) Add a new element
 addAll(c) Add a collection
clear() Remove all elements
contains(o) Membership checking
containsAll(c) Inclusion checking
isEmpty() Whether it is empty
iterator() Return an iterator
remove(o) Remove an element
removeAll(c) Remove a collection
retainAll(c) Keep the elements
size() The number of elements

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Interface List
add(i,o) Inserts o at position i.
get(i) Returns the i-th element.
remove(i) Removes the i-th element.
set(i,o) Replaces the i-th element with o.
indexOf(o) Searches object from beginning.
lastIndexOf(o) Searches object from end.
sublist(i, noOfElements) Returns sublist.
List allows multiple NULL values to be inserted.

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Interface Set
Contains UNIQUE set of values.
Contains at most one NULL value.
Does not declare additional methods.

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Interface Queue
 element(): Retrieves, but does not remove the head of this queue.
 offer(E o): Inserts the specified element into this queue, if possible. Returns
true if inserted.
 peek(): Retrieves, but does not remove the head of this queue, returning null
if this queue is empty.
 poll(): Retrieves and removes the head of this queue, or null if this queue is
empty.
 remove(): Retrieves and removes the head of this queue.
Throws exception Returns special value
Insert add(e) offer(e)
Remove remove() poll()
Examine element() peek()

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Interface Map
 clear() Removes all mappings.
 containsKey(k) Checks membership of Key.
 containsValue(v) Checks membership of Value.
 entrySet() Set of key-value pairs.
 get(k) Returns the value associated with
key.
 isEmpty() Whether it is empty.
 keySet() Returns Set of keys.
 put(k,v) Inserts Key and Value pair.
 remove(k) Removes the mapping of Key.
 size() Returns size of Map.
 values() Returns the List of values.

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Concrete Collections
Interface Set List Map
Impleme-
ntation
HashSet HashMap
ArrayList
TreeSet
(SortedSet)
TreeMap
(SortedMap)
LinkedList
Historical Vector Stack Hashtable Properties

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Concrete Collections
HashSet Set hash table
TreeSet SortedSet balanced binary tree
ArrayList List resizable-array
LinkedList List linked list
Vector List resizable-array
HashMap Map hash table
TreeMap SortedMap balanced binary tree
Hashtable Map hash table

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ArrayList implements List
public static void main(String[] args) {
o ArrayList v = new ArrayList();
o v.add(“One");
o v.add(“Two");
o v.add(“Three");
o Integer i = new Integer(4);
o v.add(i);
o Integer value = (Integer) v.get(3);
o System.out.println(“Index#3 value = " + value);

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Vector implements List
public static void main(String[] args) {
o Vector v = new Vector();
o v.add(“One");
o v.add(“Two");
o v.add(“Three");
o Integer i = new Integer(4);
o v.add(i);
o Integer value = (Integer) v.get(3);
o System.out.println(“Index#3 value = " + value);

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ArrayList – read all elements
public static void main(String[] args) {
o ArrayList v = new ArrayList();
o v.add(“One");
o …..
o int size = v.size();
o for (int j = 0; j < size; j++) {
o System.out.println(j + " : " + v.get(j));
o }
An object is converted into string by o.toString() method.
toString() method is defined in Object class.

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toString() : converts object into string
System.out.println(j + " : " + v.get(j));
OR
Object o = v.get(j);
System.out.println(j + " : " + o);
OR
Object o = v.get(j);
System.out.println(j + " : " + o.toString());

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ArrayList – read all elements
 public static void main(String[] args) {
o ArrayList v = new ArrayList();
o v.add("Jay");
o …..
o Iterator it = v.iterator(); //Gets an iterator
o while(it.hasNext())
• Object o = it.next(); //Gets next element
• System.out.println(o);
• //System.out.println(it.next());
o }
 }

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Interface Iterator
Methods
oboolean hasNext();
oObject next();
ovoid remove();
The iterator() method is defined in the
Collection interface

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Iterator
Set s = new TreeSet();
s.add(“123”); ..
Iterator it = s.iterator();
Vector v = new Vector();
v.add(“123”); ..
Iterator it = v.iterator();

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Map – Hashtable and HashMap
 public static void main(String[] args) {
o HashMap m = new HashMap();
o m.put("RN1001", 890);
o m.put("RN1002",900);
o m.put("RN1003",780);
o Integer i = (Integer)m.get("RN1002");
o Set keys = m.keySet();
o Collection vals = m.values()

23. ArrayList vs Linked List
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24. HashMap vs Hashtable
HashMap
Asynchronous.
Not Thread Safe.
Permits NULL values as
Key and Value.
High performance.
Used in single-user
application.
Hashtable
Synchronous
Thread Safe
Does not permit NULLs
Slow performance
Used in multi-user
application.
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25. ArrayList vs Vector
ArrayList
Asynchronous.
Not Thread Safe.
High performance.
Grows by half of its size.
Used in single-user
application.
Vector
Synchronous
Thread Safe
Slow performance
Double the size when grow
Used in multi-user
application.
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Grows by half of its size Grows by double of its size
+ +

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java.util.Enumeration
Vector v = new Vector();
Enumeration e = v.elements()
while (e.hasMoreElements()) {
System.out.println(e.nextElement());
}
Iterator and Enumeration both are same but Iterator has
one additional method ‘remove()’.
Enumeration can be used with only historical classes
Vector, Hashtable, etc.

27. Iterator vs Enumeration
Iterator
It is fail-fast.
Can remove an object.
Enumeration
Not fail-fast
Can’t remove an object
Available with historical
classes Vector, Hashtable,
Stack, etc.
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28. Fail-fast
If the collection is structurally modified at any
time after the Iterator is created, in any way
except through the Iterator's own remove()
method, then Iterator will throw the following
exception:
oConcurrentModificationException.
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Instantiate V/AL/HT/HM
 Vector v = new Vector(); //Default Capacity 10
 Vector v = new Vector(20); //Initial capacity 20
 Vector v = new Vector(20,15); //Initial capacity 20 and then
increase by 15
 ArrayList al = new ArrayList(); //Default Capacity 10
 ArrayList al = new ArrayList(20); //Initial Capacity
 Hashtable ht = new Hashtable();
 Hashtable ht = new Hashtable(20);
 int i = al.capacity(); //Returns the current capacity of this vector
 int s = al.size(); //Returns the number of components in this
vector

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Synchronize Collection
Non-synchronized collections can be synchronized by
Collections class using following methods:
o Collection c = Collections.synchronizedCollection(c);
o List syncList = Collections.synchronizedList(al);
o Set syncSet = Collections.synchronizedSet(s);
o SortedSet syncSet = Collections.synchronizedSortedSet(s);
o Map syncMap = Collections.synchronizedMap(m);
o TreeMap syncMap = Collections.synchronizedTreeMap(m);

31. Method equals() and hashCode()
Both methods are found in Object class.
They play imported role in identification and
searching of elements in Collections.
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32. Marksheet – Search and Remove
 List l = new ArrayList()
 Marksheet m1 = new Marksheet();
 l.add(m1);
 Marksheet m2 = new Marksheet();
 l.add(m2);
 …
 l.contains(m2); //uses equals()
 l.remove(m2) //uses equals()
 …
 HashMap map = new HashMap();
 map.put(m1,”M1”); //use hashCode()
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:Marksheet
-rollNo:String
-name : String
-physics :int
-chemistry :int
-maths :int
+getRollNo():String
+setRollNo()
+getPhysics ():int
+setPhysics ()
+getChemistry():int
+setChemistry()
+getMaths():int
+setMaths()

33. Overriding equals() and hashCode()
equals() is used to search or remove an object in a
Collection.
hashCode() returns an integer that uniquely identifies an
object in Hash Collections.
If two objects are equals by equals() method then their
hash code produced by hashCode() method must be
same.
If equals() method is overridden then it is recommended
to override hashCode() method to maintain uniqueness
on an object in the collection.
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34. Marksheet: equals() and hashCode()
Here are overridden methods of Marksheet class
public boolean equals(Object o) {
 if (o == null) return false;
 if (!(o instanceof Marksheet)) return false;
 Marksheet other = (Marksheet) o;
 return this.getRollNo().equals(other.getRollNo());
}
public int hashCode() {
 return rollNo.hashCode();
}
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35. Sorting
Framework provides two interfaces to sort the
collection.
Comparable and Comparator interfaces are used to
compare same type of objects.
Collections.sort() method uses either interface to sort
elements of a collection.
Sortable Class can implement Comparable interface
and provides natural ordering using primary key attribute.
Sortable Class may have multiple Comparators,
comparing on different attributes.
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36. Sorting ( Cont. )
A collection storing objects of Sortable Class, can be
sorted by Collections.sort() method.
// Sorts using Comparable
Collections.sort(c)
// Sorts using Comparator object
Collections.sort(c, comparator)
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37. Comparable Interface
 It does natural ordering.
 It uses primary key to order collection.
 public int compareTo(Marksheet m) {
 return rollNo.compareTo(m.rollNo);
 }
 ArrayList marksheets = new ArrayList();
 marksheets.add(new Marksheet());
 marksheets.add(new Marksheet());
 marksheets.add(new Marksheet());
 Collections.sort(marksheets);
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:Marksheet
-rollNo:String
-name : String
-physics :int
…
+compareTo(Marksheet m):int
+getRollNo():String
+setRollNo()
+getPhysics ():int
+setPhysics ()
+getChemistry():int
+setChemistry()
+…..
:Comparable

38. Comparator Interface
 One class may have multiple comparators.
 Number of comparators are depending on
number of attributes to be sorted.
 If Marskheets are sorted ascending by ‘name’
attribute then OrderByName comparator is
created.
 If Marskheets are sorted by ascending and
descending by ‘name’ attribute then
OrderByNameAsc and OrderByNameDesc
comparators are created.
 More attributes are required to be sorted,
create more comparators.
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:OrderByName
+compare(Marksheet m1,
Marksheet m2) : int
:Comparator

39. Comparator Interface ( cont.)
 class OrderByName implements Comparator<Marksheet>{
 public int compare(Marksheet m1,Marksheet m2){
 return m1.getName().compareTo(m2.getName());
 }
 }
 class OrderByMaths implements Comparator<Marksheet>{
 public int compare(Marksheet m1,Marksheet m2){
 return m1.getMaths() - m2.getMaths());
 }
 }
 Collections.sort(marksheets, new OrderByName());
 Collections.sort(marksheets, new OrderByMaths());
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40. Return value ( -ve, 0 , +ve )
Methods compare() and compareTo(), both
return integer value that can be –ve, 0 or +ve.
o -ve indicates current (first) object is smaller than
second one.
o 0 indicates both are equals.
o +ve indicates current (first) object is greater than
second one.
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Generics
Generics provides a way in order to
communicate the type of a collection to the
compiler.
Defines type of a collection to compiler.

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Before Generics
ArrayList l = new ArrayList ();
l.add("One");
l.add("Two");
String str = (String)l.get(1);
System.out.println(str);
Iterator it = l.iterator();
while (it.hasNext()) {
 String val = (String) it.next();
 ..
}

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After Generics
ArrayList<String> l = new ArrayList<String> ();
l.add("One");
l.add("Two");
String str = l.get(1);
System.out.println(str);
Iterator<String> it = l.iterator();
while (it.hasNext()) {
 String val = it.next();
 ..
}

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Generics
 Before
 HashMap map = new HashMap ();
 map.put(“1”,”One”); //Correct
 After
 HashMap <String,Integer> map
 = new HashMap <String,Integer>();
 map.put(“1”,”One”); // Compilation Error

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AutoBoxing/UnBoxing
Primitive – Wrapper Class
* byte - Byte
* short - Short
* int - Integer
* long - Long
* float - Float
* double - Double
* char - Character
* boolean - Boolean

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Wrapper   Primitive
 double d = 5.5;
 Double dObj = new Double(d);
 double d1 = dObj.doubleValue();
 char c = 'A';
 Char cObj = new Char(c);
 char c1 = cObj.charValue();
 boolean b = true;
 Boolean bObj = new Boolean(b);
 boolean b1 = bObj.booleanValue();

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Autoboxing UnBoxing
//Old
int i = 5;
Integer iObj = new Integer(i);
ArrayList l = new ArrayList();
l.add(iObj);
int k = iObj.intValue();
//New Autoboxing/UnBoxing
Integer iObj = i; //Autoboxing
int k = iObj; //UnBoxing
l.add(k); //Autoboxing

48. Disclaimer
This is an educational presentation to enhance the
skill of computer science students.
This presentation is available for free to computer
science students.
Some internet images from different URLs are
used in this presentation to simplify technical
examples and correlate examples with the real
world.
We are grateful to owners of these URLs and
pictures.
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49. Thank You!
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