question about Collections/Maps

Hi, I got this question on an interview. My gut instinct is no, but I
can't justify the reasons. Any insight is greatly appreciated.

Assume you are writing an application working with ID objects. The
application uses these ID objects in Collections, placing them into
Collection implementations and querying if IDs are in a Collection
using the method Collection.contains(). The application also uses ID
objects as keys in Maps, to associate an ID with another Object and to
efficiently lookup those Objects based on ID. Will the following
implementation of ID work in our application, given the usages listed
above? If not, please fix it so that it will. (Note: the following code
compiles fine.)

      /** An ID is just a String */
      public class ID {
          /** The ID value */
          private String _id;

          /** Construct an ID given its String value */
          public ID(String id) {
              if (id == null)
                  throw new NullPointerException();

              _id = id;
          }

          /** Get the ID value */
          public String getID() {
              return _id;
          }
      }

Thanks, -