ContactsMockTest

@isTest
private class ContactsMockTest
{
    @isTest
    private static void contactConversionTest()
    {
        // Create Mocks
        fflib_ApexMocks mocks = new fflib_ApexMocks();
        fflib_ISObjectUnitOfWork uowMock = new fflib_SObjectMocks.SObjectUnitOfWork(mocks);
        IContacts domainMock = new Mocks.Contacts(mocks);
        IAccountsSelector selectorMock = new Mocks.AccountsSelector(mocks);

        // Given
        mocks.startStubbing();

        List<Account> myAccounts = new List<Account>();
        myAccounts.add(new Account(Id = fflib_IDGenerator.generate(Account.SObjectType), Name = 'Old Account'));
        myAccounts.add(new Account(Id = fflib_IDGenerator.generate(Account.SObjectType), Name = 'New Account'));

        Id contactId = fflib_IDGenerator.generate(Contact.SObjectType);
        Contact oldCon = new Contact(
            Id = contactId,
            AccountId = myAccounts[0].Id,
            FirstName = 'OldTest',
            LastName = 'Contact'
        );
        Contact con = new Contact(
            Id = contactId,
            AccountId = myAccounts[1].Id,
            FirstName = 'NewTest',
            LastName = 'Contact'
        );

        mocks.when(selectorMock.selectById(new Set<Id>{myAccounts[0].Id})).thenReturn(myAccounts[0]);
        mocks.when(selectorMock.selectById(new Set<Id>{myAccounts[1].Id})).thenReturn(myAccounts[1]);

        mocks.stopStubbing();

        Application.UnitOfWork.setMock(uowMock);
        Application.Domain.setMock(domainMock);
        Application.Selector.setMock(selectorMock);

        // When
        IContacts contactsInstance = Contacts.newInstance(new List<Contact>{con});
        contactsInstance.conversion(new Map<Id, Contact>(new List<Contact>{oldCon}), uowMock, FALSE);
    }
}