@Override
    public URI getId() {
        URI id = super.getId();
        if (id != null) return id;

        // Fallback to index:[source]/[partid]/[id]
        StringBuilder sb = new StringBuilder(64);
        sb.append("index:").append(getSource()).append('/').append(getPartId()).append('/');
        appendAsHex(globalId, sb);
        URI indexUri = new URI(sb.toString());
        assignId(indexUri);
        return indexUri;
    }
Returns the explicitly set uri if available, returns "index:[source]/[partid]/[id]" otherwise

@return uri of hit
container-search/src/main/java/com/yahoo/prelude/fastsearch/FastHit.java