I'm upset at a janitor who whistled at me and my friend
Why would whistling make you upset?
He whistled at me to get my attention
Just because someone wanted to get your attention doesn't mean that they had evil intentions. Don't jump to conclusions. Behave in a way that is friendly and appropriate.
I'm not going to be nice to someone who whistles like that.
Why did that upset you? Did you think that their intentions were bad? If they meant well you should find out what they wanted, If they didnt, then you should talk things out with them.