This document provides information about Java collections framework. It discusses various collection interfaces like Collection, List, Set, Queue, Map and their implementations like ArrayList, LinkedList, HashSet, TreeSet, HashMap, TreeMap. It also covers topics like sorting collections using Comparable and Comparator interfaces, overriding equals() and hashCode() methods.

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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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