An example on how to setup what a mock returns using BDD syntax.

CustomerServiceTestBDD.java

@Test
public void getCustomerById_returnsCorrectCustomer_whenGivenValidId() {
    // Given
    given(mockCustomerRepository.findById(123L))
        .willReturn(new Customer(123, "Sam"));

    // When
    Customer result = customerService.getCustomerById(123);

    // Then
    assertThat(result.name).isEqualTo("Sam");
}