Method getOnlyElement

guava/src/com/google/common/collect/Iterators.java:310–327

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.

(Iterator<T> iterator)

   *     iterator is unspecified.
   */
  @ParametricNullness
  public static <T extends @Nullable Object> T getOnlyElement(Iterator<T> iterator) {
    T first = iterator.next();
    if (!iterator.hasNext()) {
      return first;
    }

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

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

  /**
   * Returns the single element contained in {@code iterator}, or {@code defaultValue} if the