/guava/guava/src/com/google/common/util/concurrent/AbstractFuture.java
this.executor = null; this.task = null;
this.task = null; this.executor = null;
@CheckForNull Listener next; Listener(Runnable task, Executor executor) { this.task = task; this.executor = executor; } Listener() { <extra_id_0> } } private static final Object NULL = new Object(); private static final class Failure { static final Failure FALLBACK_INSTANCE = new Failure(