This is very important concept in java

1. Collections frame work
The process of storing the data in the memory is known as data structures. In
which either can store the data or search the data or sort the data very efficiently.
 In java language data structures can be handled by using
“collection frameworks”
 If any object is collection of heterogeneous objects is known as
“collection object”.
 Every collection object is depends on a class known as collection
class, collection of collection classes is known as “collection
frameworks”.
 The main aim of collection frameworks is to manage the data of
heterogeneous classes very efficiently in the memory location.
 Collection framework mainly depends on 3 interfaces.
1. list interface
2. Set interface
3. Map interface.
List interface:-
It is a predefined interface in java.util.pacakge, and which was
implemented in the following classes.
1. Stack
2. Array list
3. Vector
4. Linked list
The main aim of the list interface is accept duplicate values. i.e.by
creating object of any list interface classes we can able to store any
heterogeneous values with repetition.

2. Set interface:-
it is a predefined interface in java.util.pacakge, implemented in the
following classes
1. HashSet
2. Linked HashSet
The main aim of the set interface is to accept unique values in the
memory i.e. the objects of set interface implemented classes, can
accept non repetition values.
3. Map interface:-
It is a predefined interface in java.util.pacakge and it is implemented
in the following classes
1. HashMap
2. HashTable
The main aim of the map interface is used to store data in the form of
key value pairs, in this scenario key should be the unique value, value
can be any duplicate data or repeated data.
Retrieving of data from collection object:-
The data from any collection object can be retrieved using
following 4 techniques.
1. For-each loop
2. Iterator interface
3. List IteratorInterface
4. Enumerator
Note:

3. The classes which are derived from list, set, map are technically
known as collection classes, the objects which are created for these
classes are known as collection objects.
For each loop:-
This is similar to “for loop”. Which will execute multiple times.
Whenever the collection object contains list of values otherwise the
for each loop will be terminated.
Syntax:
For(data type variable : collection object)
{
--
--
--
}
In the above syntax collection object can be object of any collection
class, it returns a value whenever it contains list of data in its memory. If
no values are available or control reaches to end of collection object
memory then it returns null value so that collection will be failed in for
each loop.
Iterator interface:-
This is alternative mechanism to retrieve the values from the collection
object, it mainly contains following methods
1. hasNext()
2. next()

4. hasNext():-
it is a predefined method used to check whether any value available in
the location in the next location of collection object if any element is
existing returns true otherwise false.
Next():-
Returns next element in the collection object.
Note
By using iterator interface we can retrieve the data only by
searching in the forward direction (from left to right)
While (iterator object.hasnext())
{
Sop(iterator object.next();
}
ListIterator interface:-
It is similar to iterator interface can be used to retrieve the data either in
forward direction or backword direction.
Mainly it contains following methods.
hasNext():-
it is predefined method returs true in the listiterator has more elements when
transeversing the list in the forward direction
Next();

5. Which returns next element which is available in the list.
hasPrevious():
it returns true if the list iterator has more elements when traversing the list in the
reverse direction
Previous():
It is predefined method returns the previous element in the list
Creation of collection object:-
For any type of collection class we must use following syntax to create an object.
Collection class<tamplate> obj=new collection class <tamplat> ();
In the above syntax collection class can be any implemented class of list/set/map
interfaces
Template can be any wrapper class, based on which the sufficient memory will be
allocated and the related data will be accepted.
Ex:-
Stack<String> s=new stack <String>();
In the above syntax stack is a collection class and whose object can hold only
string values.
List interface implemented classes:-
1.ArrayList:-
It is a predefined class in java.util.package represents a growable array. That means
whose size can be created by designing object for arraylist class.
Arraylist <tamplate> obj=new Arraylist<tamplate>();
ArrayList class contains following methods to performs various operations like
insert delete sesarching etc.
Methods of ArrayList():-
Add():-

6. It is a performed method can be used to add new element in the Array list
and it can be used in two different way.
a.add(element):-
which will be add a new element at the end of array list
b. add(index,element):-
which will add any element at a specific index values (insertion operaton)
ex:-
Arraylist<string> al =new Array list <string>();
Note:-
By default 10 memory location are given by arraylist class object. And this can be
resized at the execution time.
addAll():-
which can be used to add all the list of one Arraylist elements to another
Array list and it can be used I two difference ways.
a. addAll(collection object):
which can be used to add all the elements of given Array list at the
end of another Array list.
b. addAll(index, collection):-
which will add values of collection object at a specific index
values.

7. Array List < String > al1= new Array list <String>();
Al.addAll(al)
Al.addAll(3,al)
Remove();
Which can be used to remove an element from the Arraylist
and it can be used in 2 different ways
a. remove(element):
which removes an element based on its name
ex:-
al.remove(“india”);
b. remove (index value):-
which will remove an element based on index value.
Ex:- al.remove(1);
c. removeRamnge():-
which can be used to remove list of elements in a given range.
Sysntax:- removeRange(strting index, ending index)
d. clear():-
which can be used to remove all the elements in the given
arraylist.
Ex:- al.clear();
Size():-
Which can be used to find the no.of elements in the collecion
objects

8. Al.size(); returns no.of elements.
IsEmpty():-
It is a boolean method returns true if the array list is empty.
Otherwise reutrns false.
Al.isEmpty();
indexOf();-
which can be used to get the index value of a specific
element(always it gives index of first occurrence element).
Al.indexOf(“element”);
lastIndexOf():-
which can be used to get the index value of the last occurrence
element.
Al.lastIndexOf(“element”);
Get():-
Which can be used to retrive an element an element based on
index value.
Al.get(index value);
Contains():-
It is a boolean method returns true if the given element is
existing in the array list otherwise returns false.
Al.contains(“element”);
Clone():-
Which can be used to derive one more arraylist from the given
existing list with same elements and features.
toArray():-

9. it is a predefined method can be used to convert one arraylist
into array. i.e. growable array is converted into normal array so
that we cont perform insertion or deletion operations
sysntax:- al.toArray();
write a java program to implement arraylist.
Import java.util.*;
Class ArrayListDemo
{
Public static viod main(String []args)
{
ArrayList<string> ar1=new ArrayList<String>();
Ar1.add(“c-lang”);
Ar1.add(“c++”);
Ar1.add(“java”);
Ar1.add(“.net”);
Ar1.add(“java”);
System.out.println(“contents in ar1:”+ar1);
Ar1.add(2,”android”);
System.out.println(“contents after adding in ar1:”+ar1);
System.out.println(“…………….”);
System.out.println(“contents at first index ar1:”+ar1.get(1));
System.out.println(“…………”);
Ar1.remove(3);
Ar1.remove(“c++”);
System.out.println(“content after removing in ar1:”+ar1);
System.out.println(“size of arrayList:”+ar1.size());
System.out.println(“extrcting using for each loop”);
For (string j:ar1)

10. {
System.out.println(j);
}
ArrayList<string> ar2=new ArrayList<String>();
arl2.addAll(ar1);
System.out.println(“content in ar1:”+ar1);
Ar1.clear();
System.out.println(“content in ar1:”+ar1);
}
}
Vector class:-
Which is also similar to array list class but the main difference
is vector is sysnchronized and arraylist is unsysncronized.i.e.
arraylist object can be shared by multiple threds at a time
only.where as vector class object is not sharable by multiple
threads at a time
Stack class:-
It is a predifined class in java.util.package mainly works on
LIFO (last in first out) mechanism i.e. in which whatever the
elements we are added first which can be retrived and removed
at last.
A stack memeory can be created by creating an object for that
class
Stack<string> ar1=new Stack<String>();
Following methods can be used ti perform various oprations.

11. Empty():-
Returns true if the stack is empty otherwise returns false
Peek():-
Looks at the element at the top of the stack without removing it
form the stack
Push():-
Which can be used to push an item into the top of stack.
Pop():-
Which can be used to rmove an element from top of the stack
and it reurns the removed element.
Search():-
It is a predefined method in stack class used to search an
element in the given stack it returns -1 if no element is found
otherwise returns any positive vslue index value.
Linked list():-
It is a predefined class in java.util.package can be used to
perform various oprations on linked list. This linked list class
supports doubdly linked list
 to create a linked list we must create an object with following
syntax.
Linkedlist<template> obj=new LinkedList<template>();
 linked list class offering various opeations to perform diff
operations like adding new element, removing existing element,
searching of an element etc..
 note: refer java Api to get the complete methods list of stack vector
and linked list along with Arraylist.
Set interface imolementaion class:-

12. HashSet():-
It is a predefined class in java.util.package. can be used to perform
various operations like add remove search of objects or elements in
the collection object. memeory location of hashset collection class
and it is offereing following methods
Add(), clear(), clone(), contains(), isEmpty(), iterator(), remove(),
size().
Note:- the above methods can be used to similar Arraylist.
Iterator():-
Which returns an “iterator object” accepts only forwarding traverse
over the elements in the set.
HashSet can be created with the following syntax.
Hash<tamplate> obj=Hash<template>();
Note:- HashSet class does not allows duplicate values ans its
values will never be stored in a sequencial order.
Write a program to implement HashSet….
Import java.util.*;
Class HashSetEx
{
Public static void main(string[] args)
{
HashSet<string> hs=new HashSet<String>();
Hs.add(“india”);
Hs.add(“America”);
Hs.add(“japan”);
Hs.add(“china”);
System.out.println(“HashSet:=”+hs);

13. Hs.add(“America”);
System.out.println(“HashSet:=”+hs);
Iterator it=hs.iterator();
System.out.println(“elements using iterator”);
While(it.hashnext())
{
System.out.println(it.next());
}
}
}
Output:
Javac HashSetEx.java
Java HashSetEx
HashSet=[America, chaina, japan, india]
Hashset=[America, chaina, japan, india]
Elements using iterator:
America
China
Japan
India
Note: hashSet Class is not thread safe of unsychronized ie whose
object is shareble by multiple threads simultaneously
LinkedHashSet Class:-
linkedHasSet is similor to Hashset class available in java.util
package can be used to perform various operations on collection
objects like adding removing searching etc.

14.  the main difference b/w Hashset and LinkedHashSet is HashSet is
unsynchronized where as linkedHashSet is synchronized ie. The
object of linkedHashset is used by one thread then it doesnot allow
other thread to working on same object
 linnedHashSet is created by following syntax
linkedHashSet<template> obj=new LinkedHashSet<template>();
note: the elements of HashSet are not stored in sequence order
wher as the elements of linkedHashSet are stored in sequence
order.
Map Interface implementation classes:-
HashMap:
It is a predefined class which can be used to store
heterogenious values in the form of (key, value) pairs, key should
be unique value where as value can be duplicate.
 To create HashMap we must use the following sysntax
HashMap<template,tamplate1>obj=newHashMap<tamplate,tampl
ate1>();
 HashMap class offering various predefined methods to perform
different operations like insertion, deletion, searching etc
Put():-
Which can be used to store a (key, value) Pair in the collection
object.
Get():-
Which can be used to get the value from the collection object
based on key value

15. Remove():-
Which can be used to remove a value along with key from
collection object based on key value
Note:- refer java Api for other methods
Writer a program to implement HashMap….
Import java.util.*;
Class HashMapDemo
{
Public static void main(string[] srgs)
{
HasMap<string, long>= new HashMap<string, long>();
String name,str;
Long phno;
Scanner s=new Scanner(system.in);
While(true)
System.out.println(“………………….”);
System.out.println(“1-Enter phone number”);
System.out.println(“2-look up in the book”);
System.out.println(“3-Display names in book”);
System.out.println(“4-Exit”);
System.out.println(“enter your choice”);
Int n= s.nextInt();
Seitch(n)
{
Case:system.out.println(“enter name:”);
Name=s.next();
System.out.println(“Enter phone number”);
Phno=s.nextLong();

16. Hm.put(name,phno);
Break;
Case2: system.out.println(“enter name:”);
Names=s.next();
Name=name.trim();
Phno=hm.get(name);
System.out.println(“Enter phone number”+phno);
Break;
Case3: set<string> s1=new HashSet<string>();
S1=hm.keyset();
System.out.println(s1);
Break;
Case4: return;
}
}
}
HashTable:-
It is a predefined class in java.util package similor to HashMap
class can be used to strore the collection of objects..
 The main difference b/w HashMap and HashTable is HashMap is
unsysnchronized(not thread safe) where hashTable is
sysnchronized.

17. }
}