How to valid duplicate element in a list in java spring?

Question

I want to valid 1 form from backend in java spring boot. When the ListUser of the form is duplicated, it will send an error message and not reload the page. I have a class request

public class StoreInformationRequest {
private Boolean isReceiveReport;
private List<String> ipAddress;
private List<Integer> listUser;

public StoreInformationRequest(Boolean isReceiveReport, List<String> ipAddress, List<Integer> listUser) {
    this.isReceiveReport = isReceiveReport;
    this.ipAddress = ipAddress;
    this.listUser = listUser;
}

}

and Controller :

public String updateStore(@PathVariable("storeCode") String storeCode, @Valid StoreInformationRequest storeInformationRequests, RedirectAttributes attributes)

Answer 1

Discarding the duplicates

If you simply want to discard the duplicates, use a Set<T> instead of a List<T>. From the Set<T> documentation:

A collection that contains no duplicate elements.

To discard the duplicates, you can use:

List<Integer> numbers = Arrays.asList(1, 2, 3, 3, 4, 4);
Set<Integer> unique = new HashSet<>(numbers);

You also could use:

List<Integer> numbers = Arrays.asList(1, 2, 3, 3, 4, 4);
List<Integer> unique = numbers.stream()
        .distinct()
        .collect(Collectors.toList());

Finding the duplicates

If you have a list and want to extract the duplicates, you can use the following:

List<Integer> numbers = Arrays.asList(1, 2, 3, 3, 4, 4);
List<Integer> duplicates = numbers.stream()
        .filter(n -> Collections.frequency(numbers, n) > 1)
        .distinct()
        .collect(Collectors.toList());

Using Hibernate Validator

If you are using Hibernate Validator, a Bean Validation implementation, you could use @UniqueElements to validate that every element in the collection is unique.

To use it, ensure your list is annotated with @UniqueElements:

@Data
public class LotteryBet {

    @UniqueElements
    private List<Integer> numbers;
}

Then ensure your controller is annotated with @Validated and the lotteryBet parameter is annotated with both @RequestBody and @Valid:

@Validated
@RestController
@RequestMapping("/lottery")
public class LotteryController {

    @PostMapping(path = "/bets", consumes = MediaType.APPLICATION_JSON_VALUE)
    public ResponseEntity<Void> bet(@RequestBody @Valid LotteryBet lotteryBet) {
        ...
    }
}

A request such as the one shown below will return 400 along with some details about the error:

POST /lottery/bets HTTP/1.1
Host: example.org
Content-Type: application/json

{
  "numbers": [1, 2, 3, 3, 4, 4]
}

Answer 2

If you want distinct values in listUser you should use Set and not List.