guava/src/com/google/common/collect/Streams.java
@Beta
  public static <T> java.util.Optional<T> findLast(Stream<T> stream) {
    class OptionalState {
      boolean set = false;
      T value = null;

      void set(@Nullable T value) {
        this.set = true;
        this.value = value;
      }

      T get() {
        checkState(set);
        return value;
      }
    }
    OptionalState state = new OptionalState();

    Deque<Spliterator<T>> splits = new ArrayDeque<>();
    splits.addLast(stream.spliterator());

    while (!splits.isEmpty()) {
      Spliterator<T> spliterator = splits.removeLast();

      if (spliterator.getExactSizeIfKnown() == 0) {
        continue; // drop this split
      }

      // Many spliterators will have trySplits that are SUBSIZED even if they are not themselves
      // SUBSIZED.
      if (spliterator.hasCharacteristics(Spliterator.SUBSIZED)) {
        // we can drill down to exactly the smallest nonempty spliterator
        while (true) {
          Spliterator<T> prefix = spliterator.trySplit();
          if (prefix == null || prefix.getExactSizeIfKnown() == 0) {
            break;
          } else if (spliterator.getExactSizeIfKnown() == 0) {
            spliterator = prefix;
            break;
          }
        }

        // spliterator is known to be nonempty now
        spliterator.forEachRemaining(state::set);
        return java.util.Optional.of(state.get());
      }

      Spliterator<T> prefix = spliterator.trySplit();
      if (prefix == null || prefix.getExactSizeIfKnown() == 0) {
        // we can't split this any further
        spliterator.forEachRemaining(state::set);
        if (state.set) {
          return java.util.Optional.of(state.get());
        }
        // fall back to the last split
        continue;
      }
      splits.addLast(prefix);
      splits.addLast(spliterator);
    }
    return java.util.Optional.empty();
  }
@Beta
  public static <T> java.util.Optional<T> findLast(Stream<T> stream) {
    class OptionalState {
      boolean set = false;
      T value = null;

      void set(@Nullable T value) {
        this.set = true;
        this.value = value;
      }

      T get() {
        checkState(set);
        return value;
      }
    }
    OptionalState state = new OptionalState();

    Deque<Spliterator<T>> splits = new ArrayDeque<>();
    splits.addLast(stream.spliterator());

    while (!splits.isEmpty()) {
      Spliterator<T> spliterator = splits.removeLast();

      if (spliterator.getExactSizeIfKnown() == 0) {
        continue; // drop this split
      }

      // Many spliterators will have trySplits that are SUBSIZED even if they are not themselves
      // SUBSIZED.
      if (spliterator.hasCharacteristics(Spliterator.SUBSIZED)) {
        // we can drill down to exactly the smallest nonempty spliterator
        while (true) {
          Spliterator<T> prefix = spliterator.trySplit();
          if (prefix == null || prefix.getExactSizeIfKnown() == 0) {
            break;
          } else if (spliterator.getExactSizeIfKnown() == 0) {
            spliterator = prefix;
            break;
          }
        }

        // spliterator is known to be nonempty now
        spliterator.forEachRemaining(state::set);
        return java.util.Optional.of(state.get());
      }

      Spliterator<T> prefix = spliterator.trySplit();
      if (prefix == null || prefix.getExactSizeIfKnown() == 0) {
        // we can't split this any further
        spliterator.forEachRemaining(state::set);
        if (state.set) {
          return java.util.Optional.of(state.get());
        }
        // fall back to the last split
        continue;
      }
      splits.addLast(prefix);
      splits.addLast(spliterator);
    }
    return java.util.Optional.empty();
  }
Returns the last element of the specified stream, or {@link java.util.Optional#empty} if the
stream is empty.

<p>Equivalent to {@code stream.reduce((a, b) -> b)}, but may perform significantly better. This
method's runtime will be between O(log n) and O(n), performing better on <a
href="http://gee.cs.oswego.edu/dl/html/StreamParallelGuidance.html">efficiently splittable</a>
streams.

<p>If the stream has nondeterministic order, this has equivalent semantics to {@link
Stream#findAny} (which you might as well use).

@see Stream#findFirst()
@throws NullPointerException if the last element of the stream is null