Hi Guys..
 Here is our today's concept..

What is Collection?

Collections are Nothing but Containers by using the Collections we will store the data we will retrieve the data and we will manipulate the data.

We are having 3 types of collections in Salesforce.

•  List
•  Set 
• Map

• List:

List is a ordered data, by using the list we will allow the duplicate values.

Syntax of a list:    list<Datatype> variable = new list<datatype> ();

Here list elements can be any datatype, either primitive datatypes (integer, string, date, datetime---etc.), Subject(standard or custom object) and Collections(list,set Map).

First value in the list always store into index “0” position.

For example:
 I am storing multiple values in the list like ‘Karthik’ ,’Kumar’,’Raj’,’Rani’----,karthik will store into index zero position and kumar will store into index 1 position and Raj will store into index 2 position.

       Index 0             index 1              index 2           indexes 3

            Karthik             Kumar                  Raj                     Rani

Lists can contain any collection and can be nested within one another and become multidimensional. For example: you can have a list of lists of sets of Integers. A list can contain up to four levels of nested collections inside it, that is, a total of five levels overall.

Ex: list<list<list<list<list<Datatype>>>>>Variable = new list<list<list<list<list<Datatype>>>>> ();

List Methods:

Add:

Using Add method we will add the values to our list

Ex:         list<string> s = new list<string>{‘Karthik’,’kumar’,’karthik’,’Raj’};

                system.debug(‘$$$$$$’+s);

                Output: Karthik, kumar, karthik, Raj

Add (index, listElement):

We can add the list value into the list at specified index position.

Ex: list<string> s = new list<string>{'Karthik','kumar','karthik','Raj'};

       S.add(2,'karthik Training');

      System. Debug('$$$$$$'+s);

Output:    Karthik, kumar, karthik Training, karthik, Raj

Add All();

Add all of the elements in the specified list to the list that calls the method. Both lists must be of the same type.

              
Ex:  list<string> s = new list<string>{'Karthik','kumar','karthik','Raj'};

              System.Debug('$$$$$$'+s);

                list<string> s1 = new list<string>{'IBM’,'TCS','CSC','CTC'};

              s1.addall(s);

                System.Debug('$$$$$$'+s1);

Output: IBM, TCS, CSC, CTC, Karthik, kumar, karthik, Raj

Clear ():

By using of the clear method we will clear the values from the list.

Ex: list<string> s = new list<string>{'Karthik','kumar','karthik','Raj'};

                s.clear();

              System.Debug('$$$$$$'+s);

Output :  {}(Blank)

Clone ():

Using the clone method we will clone the values from the list.

Ex:  list<string> s = new list<string>{'Karthik','kumar','karthik','Raj'};

       list<string> s1 = s.clone();

      System.Debug('$$$$$$'+s1);

Size ():

Returns the number of elements in the list.

EX:  list<string> s = new list<string>{'Karthik','kumar','karthik','Raj'};

       list<string> s1 = s.size();

      System.Debug('$$$$$$'+s1);

Note: There is no limit on Number of items a collection can hold. But we need to check the heap size .

• Set:

Set is an UN Ordered data, set will not allow the duplicate values.

Syntax of a List:

                Set<datatype> variable = new set<datatype> ();

Important methods in Set:

Add: 
By using add method we will add the values to our list.

Ex: set<integer> s = new set<integer>{3,5,1,7,3,6,12,15,34};

                system.debug(‘$$$$$$’+s);

                Output: 1, 3, 5, 6, 7, 12, 15, 34

Add all:

By using add all method we add all values to our set.

Ex: set<integer> s = new set<integer>{3,5,1,7,3,6,12,15,34}  ;

                list<integer> slist = new list<integer>{200,201,202,203,204,205};

                s.addall(slist);
                system.debug(‘$$$$$$’+s);

Output:  1, 3, 5, 6, 7, 12, 15, 34, 200, 201, 202,203,204,205

Clear ():

By using of clear method we will clear the values from set.

Ex: set<integer> slist = new set<integer>{200,201,202,203,204,205};

                s.clear();

Output:

Clone();

By using the clone method we will clone the values from set.

syntax: set<integer> s = new set<integer>{1,5,2,4,3,5,8};

                set<integer> s1 = s.clone();

                system.debug(‘$$$$$$’+s1);

                Output: 1, 2, 3, 4, 5, 8

Size():

Using of size method we will calculate the number of components in the set.

Ex: set<integer> s = new set<integer>{1,5,2,4,3,5,8};

                system.debug(‘$$$$$$’+s.size());

                Output: 6

• Map:

Map is a combination of key value pairs, map will allow the duplicate values but it will not allow the duplicate key.

Syntax:

map<key_datatype,value_datatype> variable = new map<key_datatype,value_datatype>();

Map methods:

Clear ():

By using clear method we will clear the values from Map.

Ex:  map<string,integer> s = new map<string,integer>{‘IND’ => 91,’US’ => 1 ,’UK’ =>2 ,’USSR’ => 3,’SL’ => 2}

s.clear();

System.debug(‘$$$$’+s);

Keyset():

Using of the key set we will get only the key’s from the map.

Ex: map<string,integer> s = new map<string,integer>{‘IND’ => 91,’US’ => 1 ,’UK’ =>2 ,’USSR’ => 3,’SL’ => 2}

System.debug(‘$$$$’+ s.keyset());

Output:  IND, SL, UK, US, USSR

Values ():

 By using of values method we will add the values from map

Ex: map<string,integer> s = new map<string,integer>{‘IND’ => 91,’US’ => 1 ,’UK’ =>2 ,’USSR’ => 3,’SL’ => 2}

System.debug(‘$$$$’+ s.values());

Output:  91, 1, 2, 3, 2

Interview questions on Collections:

Q. What is a Collection? How many types of collections we have?

Ans: Collections are nothing but container, using of collections we will store the data , retrieve the data and manipulate the data.

We are having 3 types of collections in salesforce.

•   List
•   set
•   map

Q. What is the difference between list and set?

Ans:      

• list is a ordered data and list will allow the duplicate values, first value in the list always store   into index zero position.

• Set is a UN ordered data set will not allow the duplicate values.