public boolean equals(Object o) {
        if (!(o instanceof DocumentIdResponse)) {
            return false;
        }

        DocumentIdResponse docResp = (DocumentIdResponse) o;

        return super.equals(docResp) && ((documentId == null && docResp.documentId == null) ||
                (documentId != null && docResp.documentId != null && documentId.equals(docResp.documentId)));
    }
Returns the document id of this response, or null if there is none

@return the DocumentId, or null
documentapi/src/main/java/com/yahoo/documentapi/DocumentIdResponse.java