public class AccountProcessor {
    @future
    public static void countContacts(List<Id> listAccountIds){
        Map<Id,List<Contact>> accToContactsMap = new Map<Id, List<Contact>>();
        List<Account> updateAccounts = new List<Account>();
        if(listAccountIds != null){
            for(Account acc : [SELECT Id,
                               						(Select Id from Contacts)
                               			 FROM Account
                               			 WHERE id in: listAccountIds]){
                accToContactsMap.put(acc.id, acc.Contacts);
            }
        }
        for(Id accountId : accToContactsMap.KeySet()){
            Account accUpdate = new Account(id = accountId);
            accUpdate.Number_of_Contacts__c = accToContactsMap.get(accountId).size();
            updateAccounts.add(accUpdate);
        }
        update updateAccounts;
    }
    
    
    public static List<Contact> generateRandomContacts (Integer numContacts, String lastName,Id accId){
        List<Contact> contacts = new List<Contact>();
        
        for(integer i = 0; i<numContacts; i++){
            Contact c = new Contact();
            c.FirstName = 'Trail' + i;
            c.LastName = lastName + i;
            c.AccountId = accId;	
			contacts.add(c);
        }
        return contacts;
    }
    

    public static List<Account> generateRandomAccounts (Integer numAccounts){
        List<Account> accounts = new List<Account>();

        for(integer i = 0; i<numAccounts; i++){
            Account a = new Account();
            a.Name = 'Test' + i;
            accounts.add(a);
        }
        return accounts;
    }
}

@isTest
public class AccountProcessorTest {
    
    @testSetup public static void SetupTestData(){
        
        List<Account> listofAccounts = AccountProcessor.generateRandomAccounts(1);
        insert listofAccounts;
        List<Contact> listofContacts = AccountProcessor.generateRandomContacts(5, 'Test', listofAccounts.get(0).id);
        insert listofContacts;
        
    }
    @isTest
    public static void accountProcessing()
    {
        
        List<id> accIds = new List<id>();
        for(Account a : [select id from Account]){
            accIds.add(a.id);
        }
        Test.startTest();
        AccountProcessor.countContacts(accIds);
        Test.stopTest();
        Account accountAssert = [select Number_of_Contacts__c from Account where id IN :accIds LIMIT 1];
        system.assertEquals(5, Integer.valueOf(accountAssert.Number_of_Contacts__c));
    }
}