Collection in salesforce

In collection we have, 

1)List 
2)Set 
3)Map 

List==>  1)Ordered 
                  2)Can store duplicate 
                  3)We can perform dml operations on list 

Syntax: 

• list<object> listName=new list<object>();     // List of any object...
• list<String> listName=new list<String>();    // List of string...

Method available with list,

get(); // accessing element from list...

add(); // adding element to list...

size(); // checking size of list...

clear(); // clearing list...

Declaring list for account object,
==>   List<Account> accList=new List<Account>();  //List of account...


Adding elements to list,
 ==> accList.add(someinstance);  //Some object instance 


Accessing elements from list,
==> accList.get(0);  // first element


Structure of list is,

Location:   Index 0   Index 1  Index 2   Index 3
 Values:          1                   2              3               4


Set==>1)Unordered 
              2)Duplicates are not allowed in set 

Methods available with set are,
add();  // adding element to set...
remove(); // removing elemnt from set...
contains();  // checking element inside set...

Syntax: 

• Set<id> idList=new Set<id>();   //set of id's

Adding element to set,
idList.add(1);

Removing added element from Set,
idList.remove(1);

Map==> Map is used to store key value pair. 

Method available with  map are, 

Keyset();  
Values(); 
get(Keyid); 
put(key,value); 
containskey(key); 
clear();
size();
clone();
isEmpty();
remove(key);

Syntax: 

Map can be used in different ways some listed below,

• Map<string,string> mapName=new map<string,string>();  
• Map<integer, string> mapName = new Map<integer, string>();
• Map<integer, objectName> mapName = new Map<integer, objectName>();
• Map<string,list<objectName> mapName= new 
• Map<string,list<objectName> ();

Sample ex:

1)For storing contact in map with unique number.

       Map<integer, contact> mapName = new Map<integer, contact>(); 
       contact obj=[select id,name from contact limit 1];
       mapName.put(1,obj);

2)For storing Child record list related to parent record.      

      Map<String,list<contact>> accountWithContactsMap= new Map<String,list<contact>>();

      List<Account> accountList=new List<Account>();  

    accountList=[Select id,Name,(Select id,Name from Contacts) from Account];   

        for(Account obj:accountList)   
         {   
           if(obj.Contacts.size() >=1)      
            {        
              if(!accountWithContactsMap.containsKey(obj.id))  
               {       
                 accountWithContactsMap.put(obj.id,obj.Contacts); 
               }

            }

        }

To get all Contacts related to particular Account.

Account obj= new Account();

obj=[Select id,name from account limit 1];

List<Contact> conList1=new List<Contact> conList1();

Map<String,list<contact>> accountWithContactsMap1= new Map<String,list<contact>>();

  if(accountWithContactsMap.containsKey(obj.id))

    {

     conList1=accountWithContactsMap1.get(obj.id);

    }