@future Annotation In Salesforce

1.  The method annoted with @future is known as future method.
2.  Future method :
   1.  can only return void type.
   2.  must be static method.
   3.  cannot take objects and sOjects as parameter.
   4.  allow callout when callout = true is specified.(Default is callout = false).

      public class exampleClass
      {
          /**
          * Webkul Software.
          *
          * @category  Webkul
          * @author    Webkul
          * @copyright Copyright (c) 2010-2016 Webkul Software Private Limited (https://webkul.com)
          * @license   https://store.webkul.com/license.html
          */
      
          @future (callout = true)
          public static void exampleCallout()
          {
            //Write your code here.
          }
      }

3.  Specified parameter must be primitive data types, arrays of primitive data types, or collections of primitive data types.
4.  Method may or may not be executing in same order as it is called.
5.  It cannot be used in visualforce controller either in getMethodName() or in setMethodName() nor in constructor.
6.  Nested future calling is not allowed means you cannot call a future method in another future method niether in trigger.

Below is an example of future method. In this example you will create an custom field name Number_Of_Contacts___c of type number in Account sObject and update this field using future method as explained below.

1. You have to create an custom field from Setup>> Customize>>Accounts>>Fields. Create new custom field name Number_Of_Contacts__c and type should be Number.
2. Now create an apex class AccountProcessor to update the number of Contacts in each Account.

   public class AccountProcessor {
     
       /**
       * Webkul Software.
       *
       * @category  Webkul
       * @author    Webkul
       * @copyright Copyright (c) 2010-2016 Webkul Software Private Limited (https://webkul.com)
       * @license   https://store.webkul.com/license.html
       */
       
       @future
       public static void countContacts (List<Id> accountIds)
       {
           List<Account> accountList = new List<Account>();
           for(Id accId : accountIds)
           {
               Account acc = [Select Name from Account where Id =: accId];
               acc.Number_of_Contacts__c = [Select count() from Contact where AccountId =: accId];
               accountList.add(acc);
           }
           update accountList;
       }
       
       public AccountProcessor ()
       {
           List<Account> accList = [Select Id from Account];
           List<Id> accountIds = new List<Id>();
           for(Account acc: accList)
           {
               accountIds.add(acc.Id);
           }
           AccountProcessor.countContacts(accountIds);
       }
   }

3. Now open Execute Anonymous window (Ctrl+E) and execute the following code.

   AccountProcessor a = new AccountProcessor();

Now, go to Accounts and check the updated record. You will find total number of contact associated with that account in Number_Of_Account custom field.