docs: public static <T> T get(Collection<T> collection, int index) {
  		if (null == collection) {
  			return null;
  		}
  
  		final int size = collection.size();
  		if (index < 0) {
  			index += size;
  		}
  
  		// 检查越界
  		if (index >= size) {
  			return null;
  		}
  
  		if (collection instanceof List) {
  			final List<T> list = ((List<T>) collection);
  			return list.get(index);
  		} else {
  			int i = 0;
  			for (T t : collection) {
  				if (i > index) {
  					break;
  				} else if (i == index) {
  					return t;
  				}
  				i++;
  			}
  		}
  		return null;
  	}