ManualDependencyInjectionDemo.java

// please don't ever do this. 
public class ManualDependencyInjectionDemo {

    @Getter @Setter
    public static class UserService {
        public User getUser(String username){
            // ...
        }
    }

    // correlates customer details with a user account
    @Getter @Setter
    public static class CustomerService {

        private UserService userService;
        private PaymentService paymentService;
        private CustomerRepo repo;

        public CustomerDetails getDetails(String username){
            User user = userRepo.getUser(username);
            Customer customer = repo.findByUserId(user.getId());

            List<Payment> payments = paymentService.findPayments(customer.getId());
            customer.setPayments(payments);
            return customer;
        }
    }

    public static class PaymentService {
        public List<Payment> findPayments(Long customerId){
            // ..
        }
    }

    @Getter
    public static class Injector {

        private UserService userService;
        private PaymentService paymentService;
        private CustomerRepo customerRepo;
        private CustomerService customerService;

        public void init(){
            // step 1: create your instances
            userService = new UserService();
            paymentService = new PaymentService();
            customerRepo = new CustomerRepo(); // you get the idea...
            customerService = new CustomerService();

            // step 2: wire up their dependencies
            customerService.setUserService(userService);
            customerService.setPaymentService(paymentService);
            customerService.setRepo(customerRepo);

            // step 3: at this point, everything should be wired up
            // so now your app can function
        }
    }

    public static void main(String[] args){
        Injector di = new Injector();
        di.init();

        CustomerService fullyWiredService = di.getCustomerService();
        CustomerDetails details = fullyWiredService.getDetails("Tyler Evans");
        // success!
    }
}