-import com.google.common.base.Preconditions;
-import javax.annotation.Nonnull;
-import org.opendaylight.controller.cluster.raft.Snapshot;
+import static java.util.Objects.requireNonNull;
+
+import akka.dispatch.ControlMessage;
+import org.eclipse.jdt.annotation.NonNull;
+import org.opendaylight.controller.cluster.raft.persisted.Snapshot;