protected int addKey(String k) {
    Integer rv = rkeys.get(k);
    if (rv == null) {
      rv = generateOpaque();
      keys.put(rv, k);
      bkeys.put(rv, KeyUtil.getKeyBytes(k));
      rkeys.put(k, rv);
      synchronized (vbmap) {
        vbmap.put(k, new Short((short) 0));
      }
    }
    return rv;
  }
Add a key (and return its new opaque value).
src/main/java/net/spy/memcached/protocol/binary/MultiGetOperationImpl.java