Use Future Methods

Use Future Methods from (module - Asynchronous Apex)

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SOURCE CODE1:

public class AccountProcessor {
    @future
    public static void countContacts(List<Id> accountIds){
        List<Account> accounts = [Select Id, Name from Account Where Id IN : accountIds];
        List<Account> updatedAccounts = new List<Account>();
        for(Account account : accounts){
           account.Number_of_Contacts__c = [Select count() from Contact Where AccountId =: account.Id];
            System.debug('No Of Contacts = ' + account.Number_of_Contacts__c);
            updatedAccounts.add(account);
        }
    update updatedAccounts;
    } }

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Source CODE2:

@isTest
public class AccountProcessorTest {
    @isTest
    public static void testNoOfContacts(){
        Account a = new Account();
        a.Name = 'Test Account';
        Insert a;
        Contact c = new Contact();
        c.FirstName = 'Bob';
        c.LastName =  'Willie';
        c.AccountId = a.Id;
        Contact c2 = new Contact();
        c2.FirstName = 'Tom';
        c2.LastName = 'Cruise';
        c2.AccountId = a.Id;
        List<Id> acctIds = new List<Id>();
        acctIds.add(a.Id);
        Test.startTest();
        AccountProcessor.countContacts(acctIds);
        Test.stopTest();
    } }