An example of using Mockito's Answer when saving an customer.

AnswerSavedCustomer.java

@Test
public void saveCustomer_savesTheCustomerToTheDatabaseWithAnAutoGeneratedId() {
    // Arrange
    when(customerRepository.save(any()))
            .thenAnswer(savedCustomerWithAutoGeneratedId());
    Customer customer = Customer.builder()
            .firstName("Sam")
            .lastName("Adams")
            .build();

    // Act
    Customer result = customerService.saveCustomer(customer);

    // Assert
    Assertions.assertThat(result).isEqualTo(customer);
}

private Answer<Customer> savedCustomerWithAutoGeneratedId() {
    return invocation -> {
        Customer savedCustomer = invocation.getArgument(0);
        savedCustomer.setCustomerId(1L);
        return savedCustomer;
    };
}