如何比较stream中的两个列表

Question

I want to compare (assert equals) for training purposes two lists in stream. So every id from sampleUsers() with every id from body of response entity should be compared. Should I create another stream? If yes then how to use it?

@Test
  void findAllUsers() {
    when(userService.findAll()).thenReturn(sampleUsers());
    ResponseEntity<List<UserDto>> responseEntity = userController.findAll();

    //first assert
    assertEquals(responseEntity.getStatusCode().value(), 200);

    //second assert
    assertEquals(responseEntity.getBody().size(), sampleUsers().size());

    //third assert
    responseEntity.getBody().stream()
          .filter(user -> user.getId() != null)
          .filter(user -> user.getUsername() != null)
          //TODO check for each user if: 
          //     user.getId is equal to id from sampleUsers() list
          //     something like: assert equals user.getId(), sampleUsers.get(0).getId()
    }

Answer 1

There are many ways to do that assertion. You could override equals() in UserDto class.

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || this.getClass() != o.getClass()) return false;
    UserDto userDto = (UserDto) o;
    return this.id.equals(userDto.id);
}

And implement a Comparator for UserDto class. This is going to be used to sort both lists.

public class UserDtoComparatorById implements Comparator<UserDto> {
    @Override
    public int compare(UserDto o1, UserDto o2) {
        return o1.getId().compareTo(o2.getId());
    }
}

Finally.

@Test
void findAllUsers() {
    when(userService.findAll()).thenReturn(sampleUsers());
    ResponseEntity<List<UserDto>> responseEntity = userController.findAll();

    //first assert
    assertEquals(responseEntity.getStatusCode().value(), 200);

    //second assert (this is optional)
    assertEquals(responseEntity.getBody().size(), sampleUsers().size());
   
    // to prevent false negatives, in case both lists have same objects
    // but different orders 
    Comparator<UserDto> comparator = new UserDtoComparatorById();
    sampleUsers.sort(comparator);
    responseEntity.getBody().sort(comparator);

    //third assert
    assertEquals(responseEntity.getBody(), sampleUsers);

    //same assertion but using streams
    AtomicInteger i = new AtomicInteger(0);
    assertEquals(
        list1.stream()
             .filter(user -> user.equals(list2.get(i.getAndIncrement())))
             // or: filter(user -> user.getId().equals(list2.get(i.getAndIncrement()).getId()))
             .count(), 
        list1.size()
    );
}

The third assert will work because both lists are sorted and assertEquals() will invoke UserDto.equals() for each element in the same position. As UserDto.equals() is overridden to check only UserDto.id , it will check the id instead of the Object pointer (address in memory), therefore returning true.

Note that in the second alternative you can not use int i because variables inside lambda functions need to be final. A simple workaround is using AtomicInteger .

Answer 2

One way is to assert each element of the stream separately which can be done with the help of following code. You can write a function for asserting streams.

static void assertStreamEquals(Stream<User> s1, Stream<User> s2) {
    Iterator<Student> iter1 = s1.iterator(), iter2 = s2.iterator();
    while(iter1.hasNext() && iter2.hasNext())
        assertEquals(iter1.next().id, iter2.next().id);
    assert !iter1.hasNext() && !iter2.hasNext();
}

and call it by passing your two streams.

@Test
void findAllUsers() {
    when(userService.findAll()).thenReturn(sampleUsers());
    ResponseEntity<List<UserDto>> responseEntity = userController.findAll();

    //first assert
    assertEquals(responseEntity.getStatusCode().value(), 200);

    //second assert
    assertEquals(responseEntity.getBody().size(), sampleUsers().size());

    //third assert
    assertStreamEquals(responseEntity.getBody(),sampleUsers);

(Assuming User is the class name for users)