AccountRepository

package bookrental.account;

import bookrental.bookrentals.BookRentals;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.query.Param;
import org.springframework.stereotype.Repository;

import java.util.List;
import java.util.Optional;

@Repository
public interface AccountRepository {

    void deleteById(Integer id);

    List<Account> findAll();

    boolean doesAccountExistsWithGivenID(@Param("accountID") int accountID);

    Account save(Account account);

    Optional<Account> findById(int id);
}

AccountService

package bookrental.account;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.List;

@Service
public class AccountService {

    private final AccountRepository accountRepository;

    @Autowired
    public AccountService(AccountRepository accountRepository) {
        this.accountRepository = accountRepository;
    }

    public Account createAccount(Account account) {
        return accountRepository.save(account);
    }

    public Account deleteAccount(int id) {
        Account bookToDelete = accountRepository.findById(id).orElse(null);
        accountRepository.deleteById(id);
        return bookToDelete;
    }

    public List<Account> getAllAccounts() {
        return new ArrayList<>(accountRepository.findAll());
    }
}

PostgreSQLAccountRepository

package bookrental.account;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;

import java.util.List;
import java.util.Optional;

@Repository
public class PostgreSQLAccountRepository implements AccountRepository {

    private final CrudRepository<Account, Integer> repository;

    @Autowired
    public PostgreSQLAccountRepository(CrudRepository<Account, Integer> repository) {
        this.repository = repository;
    }

    @Override
    public void deleteById(Integer id) {
        this.repository.deleteById(id);
    }

    @Override
    public List<Account> findAll() {
        return (List<Account>) repository.findAll();
    }

    @Override
    public boolean doesAccountExistsWithGivenID(int accountID) {
        return repository.existsById(accountID);
    }

    @Override
    public Account save(Account account) {
        return repository.save(account);
    }

    @Override
    public Optional<Account> findById(int id) {
        return repository.findById(id);
    }
}