Builder(int capacity) {
      this.contents = new @Nullable Object[capacity];
      this.size = 0;
    }

    private void ensureRoomFor(int newElements) {
      @Nullable Object[] contents = this.contents;
      int newCapacity = expandedCapacity(contents.length, size + newElements);
      // expandedCapacity handles the overflow case
      if (contents.length < newCapacity || copyOnWrite) {