In that case, you could make the ball bearing actually on the telescoping part and able to push it down with your finger while pulling the extension out. Or, you push in on the ball bearing and twist the extension, effectively lcoking it down. When pull the extension out, you just twist back to lock the ball bearing in place.

Ok, so the idea needs work, but I'm procrastinating on my java assignment and I'd rather thing about boats than code.