K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractMap<K, V> {
    /**
     * Creates the entry set to be returned by {@link #entrySet()}. This method is invoked at most
     * once on a given map, at the time when {@code entrySet} is first called.
     */
    abstract Set<Entry<K, V>> createEntrySet();

    @LazyInit @CheckForNull private transient Set<Entry<K, V>> entrySet;