import com.google.common.util.concurrent.ListeningExecutorService;
import com.google.common.util.concurrent.SettableFuture;
import com.google.common.util.concurrent.UncheckedExecutionException;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;