Method union

corpus/java/training/guava/collect/Sets.java:590–630

Returns an unmodifiable view of the union of two sets. The returned set contains all elements that are contained in either backing set. Iterating over the returned set iterates first over all the elements of set1, then over each element of set2, in order, that is not contained

(final Set<? extends E> set1, final Set<? extends E> set2)

   * the {@link Map#keySet} of an {@code IdentityHashMap} all are).
   */
  public static <E> SetView<E> union(final Set<? extends E> set1, final Set<? extends E> set2) {
    checkNotNull(set1, "set1");
    checkNotNull(set2, "set2");

    final Set<? extends E> set2minus1 = difference(set2, set1);

    return new SetView<E>() {
      @Override
      public int size() {
        return IntMath.saturatedAdd(set1.size(), set2minus1.size());
      }

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

      @Override
      public Iterator<E> iterator() {
        return Iterators.unmodifiableIterator(
            Iterators.concat(set1.iterator(), set2minus1.iterator()));
      }

      @Override
      public boolean contains(Object object) {
        return set1.contains(object) || set2.contains(object);
      }

      @Override
      public <S extends Set<E>> S copyInto(S set) {
        set.addAll(set1);
        set.addAll(set2);
        return set;
      }

      @Override
      public ImmutableSet<E> immutableCopy() {
        return new ImmutableSet.Builder<E>().addAll(set1).addAll(set2).build();
      }
    };
  }

  /**
   * Returns an unmodifiable <b>view</b> of the intersection of two sets. The