Asynchronous Apex-Use Future Methods

Challenge 7:
Create an Apex class that uses the @future annotation to update Account records.
Create an Apex class with a method using the @future annotation that accepts a List of Account IDs and updates a custom field on the Account object with the number of contacts associated to the Account. Write unit tests that achieve 100% code coverage for the class.
Create a field on the Account object called 'Number_of_Contacts__c' of type Number. This field will hold the total number of Contacts for the Account.
Create an Apex class called 'AccountProcessor' that contains a 'countContacts' method that accepts a List of Account IDs. This method must use the @future annotation.
For each Account ID passed to the method, count the number of Contact records associated to it and update the 'Number_of_Contacts__c' field with this value.
Create an Apex test class called 'AccountProcessorTest'.
The unit tests must cover all lines of code included in the AccountProcessor class, resulting in 100% code coverage.
Run your test class at least once (via 'Run All' tests the Developer Console) before attempting to verify this challenge.

Solution:
1.AccountProcessor.apxc
public class AccountProcessor {
    @future
    public static void countContacts(Set<Id> setId){
        List<Account> lstAccount = [select Id,Number_of_Contacts__c,(select id from contacts) from account where id in :setId];
        for(Account acc : lstAccount){
            List<Contact> lstCont = acc.contacts;
            acc.Number_of_Contacts__c = lstCont.size();
        }
        update lstAccount;
    }
}

2.AccountProcessorTest.apxc

@isTest
public class AccountProcessorTest {
    public static testMethod void testAccountProcessorTest(){
        Test.startTest();
        Account a = new Account();
        a.Name = 'The Pirates';
        insert a;

        Contact cont = new Contact();
        cont.FirstName ='jack';
        cont.LastName ='Sparrow';
        cont.AccountId = a.Id;
        insert cont;

        Set<Id> setAccId = new Set<ID>();
        setAccId.add(a.Id);

        AccountProcessor.countContact(setAccId);       

        Account acc = [select Number_of_Contacts__c from Account where id = :a.id LIMIT 1];
        System.assertEquals(Integer.valueOf(acc.Number_of_Contacts__c) ,1);
        Test.stopTest();
    } 
}