Method copyOf

guava/src/com/google/common/collect/ImmutableSet.java:172–214

Returns an immutable set containing each of {@code elements}, minus duplicates, in the order each appears first in the source collection. <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation is unnecessary; for example, {@code copyOf(copyOf(anArrayList))}

(Collection<? extends E> elements)

   * @since 7.0 (source-compatible since 2.0)
   */
  public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
    /*
     * TODO(lowasser): consider checking for ImmutableAsList here
     * TODO(lowasser): consider checking for Multiset here
     */
    // Don't refer to ImmutableSortedSet by name so it won't pull in all that code
    if (elements instanceof ImmutableSet && !(elements instanceof SortedSet)) {
      @SuppressWarnings("unchecked") // all supported methods are covariant
      ImmutableSet<E> set = (ImmutableSet<E>) elements;
      if (!set.isPartialView()) {
        return set;
      }
    } else if (elements instanceof EnumSet) {
      EnumSet<?> clone = ((EnumSet<?>) elements).clone();
      ImmutableSet<?> untypedResult = ImmutableEnumSet.asImmutable(clone);
      /*
       * The result has the same type argument we started with. We just couldn't express EnumSet<E>
       * or ImmutableEnumSet<E> along the way because our own <E> isn't <E extends Enum<E>>.
       *
       * We are also performing a safe covariant cast to change <? extends E> to <E>.
       */
      @SuppressWarnings("unchecked")
      ImmutableSet<E> result = (ImmutableSet<E>) untypedResult;
      return result;
    }

    if (elements.isEmpty()) {
      // We avoid allocating anything.
      return of();
    }
    // Collection<E>.toArray() is required to contain only E instances, and all we do is read them.
    // TODO(cpovirk): Consider using Object[] anyway.
    @SuppressWarnings("unchecked")
    E[] array = (E[]) elements.toArray();
    /*
     * For a Set, we guess that it contains no duplicates. That's just a guess for purpose of
     * sizing; if the Set uses different equality semantics, it might contain duplicates according
     * to equals(), and we will deduplicate those properly, albeit at some cost in allocations.
     */
    int expectedSize =
        elements instanceof Set ? array.length : estimatedSizeForUnknownDuplication(array.length);
    return fromArrayWithExpectedSize(array, expectedSize);
  }

  /**
   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order