[quote=@Crew]
Everyone should subscribe to YouTube.com/user/LegendBegins
[/quote]

I agree.

You could always just use
Collections.reverse(bars);
and finish the method in one line.
But if you want to develop an algorithm, yours currently swaps the value of k and l. You're going to have to switch the values with
bars.set(k, bars.get(l));
A more concise way to do it would be

private void reversal()
{
	int size = bars.size();
	for(int i = 0; i < size / 2; i++)
	{
		int temp = bars.get(i);
		bars.set(i, bars.get(size - 1 - i));
		bars.set(size - 1 - i, temp);
	}
}