Mockito TDD Demo

import java.time.LocalDateTime;
import java.time.Month;

import org.junit.Before;
import org.junit.Test;
import org.mockito.Mockito;

public class TodoItemServiceTest {
    private static final long ITEM_ID = 3L;

    static class TodoItem {
        private final String description;
        private final LocalDateTime deadline;
        private final long id;

        public TodoItem(long id, String description, LocalDateTime deadline) {
            super();
            this.description = description;
            this.deadline = deadline;
            this.id = id;
        }

        public String getDescription() {
            return description;
        }

        public LocalDateTime getDeadline() {
            return deadline;
        }

        public long getId() {
            return this.id;
        }
    }

    static interface TodoItemStorage {
        // TodoItem createNew(String description, LocalDateTime deadline);
        TodoItem getById(long id) throws UnknownIdException;
        void updateDeadline(long itemId, LocalDateTime newDeadline);
        // List<TodoItem> findWithDeadlineBefore(LocalDateTime latestDeadline);
        // void delete(long id);
    }

    static class UnknownIdException extends Exception {
        public UnknownIdException(long id) {
            super("Unknown id: " + id);
        }
    }

    static class TodoItemService {
        private final TodoItemStorage storage;

        public TodoItemService(TodoItemStorage storage) {
            super();
            this.storage = storage;
        }

        public void delayByOneDay(long itemId) throws UnknownIdException {
            TodoItem item = this.storage.getById(itemId);
            if(null != item) {
                LocalDateTime deadline = item.getDeadline();
                this.storage.updateDeadline(itemId, deadline.plusDays(1));
            }
        }
    }

    TodoItemStorage storage;
    TodoItemService underTest;

    @Before
    public void setUp() {
        this.storage = Mockito.mock(TodoItemStorage.class);
        this.underTest = new TodoItemService(storage);
    }

    @Test
    public void can_delay_by_one_day() throws UnknownIdException {
        // Given:
        LocalDateTime originalDeadline = LocalDateTime.of(2016, Month.MARCH,
                20, 8, 00);
        // Stub the call to getById to return a known value
        Mockito.when(storage.getById(ITEM_ID)).thenReturn(
                new TodoItem(ITEM_ID, "Blog article", originalDeadline));

        // When:
        this.underTest.delayByOneDay(ITEM_ID);

        // Then:
        // Verify that the storage interface is called with the correct arguments
        Mockito.verify(this.storage).updateDeadline(ITEM_ID,
                LocalDateTime.of(2016, Month.MARCH, 21, 8, 00));
    }

    @Test(expected=UnknownIdException.class)
    public void should_pass_on_unknown_id_exception() throws UnknownIdException {
        // Given:
        Mockito.when(storage.getById(ITEM_ID)).thenThrow(new UnknownIdException(3L));

        // When:
        this.underTest.delayByOneDay(ITEM_ID);

        // Then: expect exception
    }
}