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Collections in Salesforce

Collections:- Collections are the group of similar types Apex has the following types of Collections

  • Lists
  • Maps
  • Sets

There is no limit on the number of items a collection can hold However there is a general limit on heap size.


A list is an ordered collection of typed primitives SObjects, user-defined objects, Apex objects or collections that are distinguished by their includes.

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  • The index position of the first element in a list is always ‘0’
  • To declare a list, use the list keyword followed by the primitive data, SObject, nested list, map, or set type within < > characters

List< String > Mylist= new list<string>();

This will store the block of memory at the database

List <List<set<Integer>>>mylist2= new list <list<set<Integer>>>(); Nested List

Note:- A list can only contain up to five levels of nested collections inside it.

*The following example describes us about the retrieving 100 accounts stored in variable.

List<Account> accs= select Id,Name FROM Account limit 100 ;

*To access elements in a list, use the system methods provided by Apex.

Ex:- List<Integer> Li=new list<Integer>(); Define a new List

Li.add(47);  adds a second element of value 47 to the end of the list

Integer x= Li.get(1);Retrieves the element at index 1.

Li.Set(0,73); adds the integer 73 to the list at index o

Li.clear();  —>  removes all elements from the list                                

*Go to the Developer console and execute the following code and check the result by going through USER-DEBUG

List<String> Li=new List<string>();




System.debug(Li);  à the o/p from this line is

Anail Laxman vissu Ramu Sita

0              2            3         4       5

String name = Li.get (0);
System.debug (name); ---> the o/p from this line is ANIL
Li.set(2, ‘sanjeev’);
System debug (Li);  --->      the o/p from this line is
Li.clear ();   --->   clear all elements in a list
System.debug (li);  ---> the o/p from this line is null.

Note:- In a list, we can add Duplicate values that is duplicate values will be allowed inside a list.

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A set is an unordered collection of primitives or SObjects that do not contain any duplicate elements.To declare a set, use the set keyword followed by the primitive data type name within < > characters


set <string> s= new set < string > ();
To access elements in a set use the system methods provided by Apex.
<Integer> s=new set <Integer>(); --->   Define a new set
S.add (1);    --->  Adding an element to the set
Boolean b= s.contains(1);    ---> It will returns true when the element ‘1’ contains within the set.
s.remove (1);  ---> Remove the element from the set
*Go to the Developer console and execute the following code and check the result by going through USER-DEBUG();
Set <string> s=new set <string>(); s.add(‘red’);
system.debug (s);
This statement gives the o/p below
{Black, blue ,green ,red ]
s.remove ('green'); system.debug (s);
This statement gives the Output as below {Black, blue, red}
s.add ('red'); system.debug(s)

#### This statement gives the o/p as below

{black, blue, red} because the set will not allow duplicate values inside it Boolean b= s.contains (‘black’)

###Returns true when the value ‘black’ is within set and it returns “False” when the value ‘black’ is not there within set.

*Uniqueness of sobjects is determined by comparing fields

            EX:-If we try to add two accounts with the same name to a set,only one is added.

//Create two accounts a1 and a2
Account a1=new account (Name='Test Account');
Account a2=new account (Name='Test Account');
//Add both accounts to the new set
set<Account> acc set = new set <Account > {a1,a2};
system.assertEquals (accset.size(),1);

This statement is used to verify the set only contains one item.


A map is a collection of key-value pairs where each unique key maps to a single value

*Keys can be any primitive datatype while values can be primitive, sobject, collection type or Apex object.

*To declare a map, use the Map keyword followed by the data types of the value within < > characters


Map < string, string> m=new map <string, string> ();
Map<ID, set<string>> m1=new Map<ID, set <string>>();
Map<ID, Map <ID,Account []>> m2=new map<ID,Map<ID,Account[]>>m2;
Similar to lists, map values can contain any collection and can be nested within one another. A map can only contain up to five levels of nested collections inside it.

The following example gives the information about how a map of contain and currencies
Country ---->  Key
currency---> Value
Map<string, string> m= new Map<string> ();
m.put (‘India’, 'Rupee’);
m.put (‘us’, 'Dollar');    Insert a new key value pair in the map
m.put (‘uk’, 'Pounds');
system.debug (m);          the o/p of this line will be
{India= Rupee, us=Dolar, uk= pounds}
string currency = m.get('uk'); àRetrieve a value given a particular key system.debug (currency), à the o/p of this will be pounds
system.debug (m.keyset ()); à Return a set that contains all of the keys in the map.
system.debug (m.containkey ('uk'));

This statement gives the true or false whether for particular key have a value or not.

*   EX:- Account a= new Account (Name=’capital’,Bullingcity=’Hyd’); Map<Integer, Account> map1=new map<integer, Account> (); map1.put (1, a); àInsert a new key value pair in the map Map<Integer, Account >map2=map1.clone ();

###Makes a duplicate copy of the map system.debug ((map1.get (1) Billingcity));

###This statement gives the Billingcity filed value of the value pair that is assigned to the key 1.


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