hi @xiger
There is no way to do a rebalance operation from the Java SDK (you can only configure buckets and add/remove buckets from the SDK, not act on the nodes themselves).
However, Couchbase has an extensive REST API that you can also use to rebalance the cluster (note that you may have to look at adding / removing nodes via the REST API as well as a prerequisite to rebalancing the cluster, as stated in the doc).