Finding out if a list of Objects contains something with a specified field value?

Question

I have a list of DTO received from a DB, and they have an ID. I want to ensure that my list contains an object with a specified ID. Apparently creating an object with expected fields in this case won't help because contains() calls for Object.equals(), and they won't be equal.

I came up to a solution like so: created an interface HasId, implemented it in all my DTOs, and inherited ArrayList with a new class that has contains(Long id) method.

public interface HasId {     void setId(Long id);     Long getId(); }  public class SearchableList<T extends HasId> extends ArrayList<T> {     public boolean contains(Long id) {         for (T o : this) {             if (o.getId() == id)                 return true;         }         return false;     } } 

But in this case I can't typecast List and ArrayList to SearchableList... I'd live with that, but wanted to make sure that I'm not inventing the wheel.

EDIT (Oct '16):

Of course, with the introduction of lambdas in Java 8 the way to do this is straightforward:

list.stream().anyMatch(dto -> dto.getId() == id); 

Answer 1

I propose to create simple static method like you wrote, without any additional interfaces:

public static boolean containsId(List<DTO> list, long id) {     for (DTO object : list) {         if (object.getId() == id) {             return true;         }     }     return false; }