An example on how to setup what a mock returns.

CustomerServiceTest.java

@Test
public void getCustomerById_returnsCorrectCustomer_whenGivenValidId() {
    // Arrange
    when(mockCustomerRepository.findById(123))
        .thenReturn(new Customer(123, "Sam"));

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

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