code: protected boolean addElem(final T element) {
      // validate element
      if (element == null) {
        throw new IllegalArgumentException("Null element is not supported.");
      }
      // find hashCode & index
      final int hashCode = element.hashCode();
      final int index = getIndex(hashCode);
      // return false if already present
      if (containsElem(index, element, hashCode)) {
        return false;
      }
  
      modification++;
      size++;
  
      // update bucket linked list
      DoubleLinkedElement<T> le = new DoubleLinkedElement<T>(element, hashCode);
      le.next = entries[index];
      entries[index] = le;
  
      // insert to the end of the all-element linked list
      le.after = null;
      le.before = tail;
      if (tail != null) {
        tail.after = le;
      }
      tail = le;
      if (head == null) {
        head = le;
      }
      return true;
    }
docs: Add given element to the hash table
  
  @return true if the element was not present in the table, false otherwise