The CustomerDAO interface should have the following API:

void saveCustomer(Customer customer);
Customer getCustomer(String username);
Boolean validateCredentials(String username, String password);

It should be reasonably obvious what these methods are going to be used for. Create an interface named CustomerDAO in the dao package that has this API.

Create a collections-based DAO named CustomerCollectionsDAO that implements the interface. To save you some time, the complete collections DAO is available in the useful-files repository on GitBucket.