InvocationHandler for fake implementation

incoationHandler.java

public static CustomerRepository fakeCustomerRepository() {
    Customer alice = new Customer(1L, "Alice");
    Customer bob   = new Customer(2L, "Bob");

    List<Customer> all = List.of(alice, bob);

    return (CustomerRepository) Proxy.newProxyInstance(
        CustomerRepository.class.getClassLoader(),
        new Class[]{CustomerRepository.class},
        (proxy, method, args) -> switch (method.getName()) {
            case "findAll"  -> all;
            case "count"    -> (long) all.size();
            case "save"     -> args[0]; // visszaadja amit kapott
            case "deleteById" -> {
                System.out.println("deleteById called with: " + args[0]);
                yield null;
            }
            case "findById" -> {
                Long id = (Long) args[0];
                yield all.stream()
                         .filter(c -> c.getId().equals(id))
                         .findFirst();
            }
            case "findByName" -> {
                String name = (String) args[0];
                yield all.stream()
                         .filter(c -> c.getName().equals(name))
                         .toList();
            }
            default -> throw new UnsupportedOperationException("Not faked: " + method.getName());
        }
    );
}