The correct way to return the only element from a set

You can use an Iterator to both obtain the only element as well as verify that the collection only contains one element (thereby avoiding the size() call and the unnecessary list creation):

Iterator<Element> iterator = set.iterator();

if (!iterator.hasNext()) {
    throw new RuntimeException("Collection is empty");
}

Element element = iterator.next();

if (iterator.hasNext()) {
    throw new RuntimeException("Collection contains more than one item");
}

return element;

You would typically wrap this up in its own method:

public static <E> E getOnlyElement(Iterable<E> iterable) {
    Iterator<E> iterator = iterable.iterator();

    // The code I mentioned above...
}

Note that this implementation is already part of Google’s Guava libraries (which I highly recommend, even if you don’t use it for this particular code). More specifically, the method belongs to the Iterables class:

Element element = Iterables.getOnlyElement(set);

If you’re curious about how it is implemented, you can look at the Iterators class source code (the methods in Iterables often call methods in Iterators):

  /**
   * Returns the single element contained in {@code iterator}.
   *
   * @throws NoSuchElementException if the iterator is empty
   * @throws IllegalArgumentException if the iterator contains multiple
   *     elements.  The state of the iterator is unspecified.
   */
  public static <T> T getOnlyElement(Iterator<T> iterator) {
    T first = iterator.next();
    if (!iterator.hasNext()) {
      return first;
    }

    StringBuilder sb = new StringBuilder();
    sb.append("expected one element but was: <" + first);
    for (int i = 0; i < 4 && iterator.hasNext(); i++) {
      sb.append(", " + iterator.next());
    }
    if (iterator.hasNext()) {
      sb.append(", ...");
    }
    sb.append('>');

    throw new IllegalArgumentException(sb.toString());
  }