What is a better way to design an API for mutually exclusive request parameters?

Question

What is a better way to design an API on a @Controller method when request parameters are mutually exclusive?

Let's say that there is an API to provide a List of Users that matches the request parameters.

Code is:

public ResponseEntity getList(@RequestParam(required = false) Integer userId,
                              @RequestParam(required = false) User.Type userType,
                              @RequestParam(required = false) Integer age) {
        List<User> userList = null;
        if (userId != null) {
            //logic
            userList = getUserByUserId()
        } else if (userType != null) {
            //logic
            userList = getUserByType()
        } else if (age != null) {
            //logic
            userList = getListByAge()
        } else {
            userList = getAllWithoutCondition();
        }
        return ResponseEntity.ok(userList);
}

Here is the point:

The user cannot query with more than one request parameter. Only one request parameter or no request parameter is valid (only one of userId, age, or type should exist in a request).

I am not sure what the better way to design an API for this situation is. Can you give me some advice?

Answer 1

I like the approach suggested by the guys in the comments:

@RequestMapping(value = "...", params = {"!userType", "!userAge"})
public ResponseEntity<List<User>> getListByUserId(@RequestParam Integer userId) { ... }

// similarly, define two more

It looks robust and feasible until you start managing restrictions for each endpoint. It looks tedious and hard to maintain. Furthermore, I am not sure how the endpoint that takes no params would react. Would it get called or be shadowed by other methods?

Instead of writing restrictions, I suggest introducing conditions - the requirements for each endpoint. It could be a Map<String, Function<String, List<User>>> in the next format:

<param name> -> <action to get a list>

I also advise you collect all the incoming request params into a single Map<String, String> to validate it by size.

public class Controller {

    private Map<String, Function<String, List<User>>> handlers = new HashMap<>();

    {
        handlers.put("userId", id -> getUsersById(Integer.valueOf(id)));
        handlers.put("userType", type -> getUsersByType(User.Type.valueOf(type)));
        handlers.put("userAge", age -> getUsersByAge(Integer.valueOf(age)));
    }

    @RequestMapping("...")
    public ResponseEntity<List<User>> getList(@RequestParam Map<String, String> params) {
        if (params.size() > 1) {
            return ResponseEntity.unprocessableEntity().build();
        }

        if (params.size() == 0) {
            return ResponseEntity.ok(getAllWithoutCondition());
        }

        Map.Entry<String, String> paramEntry = params.entrySet().iterator().next();

        return ResponseEntity.ok(handlers.get(paramEntry.getKey()).apply(paramEntry.getValue()));
    }

    private List<User> getAllWithoutCondition() { ... }

    private List<User> getUsersById(Integer id) { ... }
    private List<User> getUsersByType(User.Type type) { ... }
    private List<User> getUsersByAge(Integer age) { ... }

}