public String getDisplayId() {
        String id = null;

        Object idField = getField(DOCUMENT_ID);
        if (idField != null)
            id = idField.toString();
        if (id == null)
            id = getId() == null ? null : getId().toString();
        return id;
    }
Returns the id to display, or null to not display (render) the id.
This is useful to avoid displaying ids when they are not assigned explicitly
but are just generated values for internal use.
This default implementation returns the field DOCUMENT_ID if set,
and {@link #getId()}.toString() otherwise.
container-search/src/main/java/com/yahoo/search/result/Hit.java