import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * An abstract implementation of {@link ListenableFuture}, intended for advanced users only. More
 * common ways to create a {@code ListenableFuture} include instantiating a {@link SettableFuture},
 * submitting a task to a {@link ListeningExecutorService}, and deriving a {@code Future} from an