@Override
	public TreeNode<T> childAt(final int index) {
		if (_children == null) {
			throw new ArrayIndexOutOfBoundsException(format(
				"Child index is out of bounds: %s", index
			));
		}

		return _children.get(index);
	}
Returns the child at the specified index in this node's child array.

@param index   an index into this node's child array
@return the tree-node in this node's child array at the specified index
@throws ArrayIndexOutOfBoundsException  if the {@code index} is out of
        bounds
jenetics.ext/src/main/java/io/jenetics/ext/util/TreeNode.java