为具有组合的类编写测试的最佳实践是什么

Question

Assume I have class Formater

class Formatter {
  public FormattedData format(Map<String, Data> data) {
    return .....
  }  
}

and another class Collector that uses Formatter and can return formatted data

class Collector {
  Formatter formatter;
  Map<Id, Data> map = new HashMap<>()

  class Collector (Formatter formatter) {
    this.formatter = formatter;  
  }

  public void addData(Data data) (
    map.put(data.getId(), data);
  }

  public FormattedData getFormattedData() {
    return formatter.format(map)
  }

So the problem - I want to write tests. I wrote all the tests for Formatter class, but how should I test Collector ?

Since I should not rely on implementation of collector - I need to copy all tests for Formatter and pass them as input for Collector . Surely in tests I would change Map<String, Data> data to Data data as input data type, but anyway there will be a huge code duplication. How can I avoid it?

Answer 1

You use mocks so you don't depend on the formatter implementation.

@Test
@ExtendWith(MockitoExtension.class) // @RunWith(MockitoJUnitRunner.class) for JUnit 4
class CollectorTest {
    @InjectMocks
    private Collector sut;

    @Mock
    private Formatter formatter;

    public FormattedData getFormattedData() {
        FormattedData formatted = mock(FormattedData.class);
        when(formatter.format(any()).thenReturn(formatted);

        FormattedData result = sut.getFormattedData();

        // verify injected formatter was called
        verify(formatter).format(any());
        // verify the result of the formatter is returned by the collector
        assertThat(result).isSameAs(formatted);
    }
}