android/guava/src/com/google/common/collect/Sets.java
public static <E> SetView<E> symmetricDifference(
      final Set<? extends E> set1, final Set<? extends E> set2) {
    checkNotNull(set1, "set1");
    checkNotNull(set2, "set2");

    return new SetView<E>() {
      @Override
      public UnmodifiableIterator<E> iterator() {
        final Iterator<? extends E> itr1 = set1.iterator();
        final Iterator<? extends E> itr2 = set2.iterator();
        return new AbstractIterator<E>() {
          @Override
          public E computeNext() {
            while (itr1.hasNext()) {
              E elem1 = itr1.next();
              if (!set2.contains(elem1)) {
                return elem1;
              }
            }
            while (itr2.hasNext()) {
              E elem2 = itr2.next();
              if (!set1.contains(elem2)) {
                return elem2;
              }
            }
            return endOfData();
          }
        };
      }

      @Override
      public int size() {
        int size = 0;
        for (E e : set1) {
          if (!set2.contains(e)) {
            size++;
          }
        }
        for (E e : set2) {
          if (!set1.contains(e)) {
            size++;
          }
        }
        return size;
      }

      @Override
      public boolean isEmpty() {
        return set1.equals(set2);
      }

      @Override
      public boolean contains(Object element) {
        return set1.contains(element) ^ set2.contains(element);
      }
    };
  }
public static <E> SetView<E> symmetricDifference(
      final Set<? extends E> set1, final Set<? extends E> set2) {
    checkNotNull(set1, "set1");
    checkNotNull(set2, "set2");

    return new SetView<E>() {
      @Override
      public UnmodifiableIterator<E> iterator() {
        final Iterator<? extends E> itr1 = set1.iterator();
        final Iterator<? extends E> itr2 = set2.iterator();
        return new AbstractIterator<E>() {
          @Override
          public E computeNext() {
            while (itr1.hasNext()) {
              E elem1 = itr1.next();
              if (!set2.contains(elem1)) {
                return elem1;
              }
            }
            while (itr2.hasNext()) {
              E elem2 = itr2.next();
              if (!set1.contains(elem2)) {
                return elem2;
              }
            }
            return endOfData();
          }
        };
      }

      @Override
      public int size() {
        int size = 0;
        for (E e : set1) {
          if (!set2.contains(e)) {
            size++;
          }
        }
        for (E e : set2) {
          if (!set1.contains(e)) {
            size++;
          }
        }
        return size;
      }

      @Override
      public boolean isEmpty() {
        return set1.equals(set2);
      }

      @Override
      public boolean contains(Object element) {
        return set1.contains(element) ^ set2.contains(element);
      }
    };
  }
Returns an unmodifiable <b>view</b> of the symmetric difference of two sets. The returned set
contains all elements that are contained in either {@code set1} or {@code set2} but not in
both. The iteration order of the returned set is undefined.

<p>Results are undefined if {@code set1} and {@code set2} are sets based on different
equivalence relations (as {@code HashSet}, {@code TreeSet}, and the keySet of an {@code
IdentityHashMap} all are).

@since 3.0