code: public final <K> ImmutableMap<K, E> uniqueIndex(Function<? super E, K> keyFunction) {
      return Maps.uniqueIndex(getDelegate(), keyFunction);
    }
docs: Returns a map with the contents of this {@code FluentIterable} as its {@code values}, indexed
  by keys derived from those values. In other words, each input value produces an entry in the
  map whose key is the result of applying {@code keyFunction} to that value. These entries appear
  in the same order as they appeared in this fluent iterable. Example usage:
  
  <pre>{@code
  Color red = new Color("red", 255, 0, 0);
  ...
  FluentIterable<Color> allColors = FluentIterable.from(ImmutableSet.of(red, green, blue));
  
  Map<String, Color> colorForName = allColors.uniqueIndex(toStringFunction());
  assertThat(colorForName).containsEntry("red", red);
  }</pre>
  
  <p>If your index may associate multiple values with each key, use {@link #index(Function)
  index}.
  
  <p><b>{@code Stream} equivalent:</b> {@code
  stream.collect(ImmutableMap.toImmutableMap(keyFunction, v -> v))}.
  
  @param keyFunction the function used to produce the key for each value
  @return a map mapping the result of evaluating the function {@code keyFunction} on each value
  in this fluent iterable to that value
  @throws IllegalArgumentException if {@code keyFunction} produces the same key for more than one
  value in this fluent iterable
  @throws NullPointerException if any element of this iterable is {@code null}, or if {@code
  keyFunction} produces {@code null} for any key
  @since 14.0