code: @Beta
      public static <K, V> Map.Entry<K, V> getOnlyEntry(Map<K, V> map) {
          checkArgument(map != null, "Expected non-null map");
          return getOnlyElement(map.entrySet());
      }
docs: Gets the only element from a given map or throws an exception
  
  @param map the map to get only entry from
  @param <K> the key type
  @param <V> the value type
  @return the single entry contained in the map
  @throws NoSuchElementException   if the map is empty
  @throws IllegalArgumentException if the map contains multiple entries
  @see com.google.common.collect.Iterables#getOnlyElement