This salad is more than the sum of its parts. The tartness of the lemon and the sweetness of the corn and avocado go really well together, and the beans provide an earthy backdrop. It's so simple that all the flavors are pure and present. And while it can serve 2-3 people as a side, I will happily eat most of it as my lunch.
I started making this many years ago when I was in graduate school; it makes a great lunch, either by itself or wrapped in a tortilla, and can be prepared directly in a jar to take with you. It can withstand a few hours in a fridge, but I wouldn't push past that. If you're serving it as part of a meal, it will be at its freshest and best right away (for ~20 minutes).
Variation: add a titch of fresh hot pepper, minced, to make it more of a salsa. If you're using canned beans, I recommend Goya's. —booglix
Get the corn ready: if fresh, steam it lightly until tender and then let it cool a few minutes; if frozen, defrost to room temperature. You can accomplish either task quickly by putting the corn in a bowl with a sprinkle of water, covering with a plate, and microwaving (check often), then draining and cooling.
Juice the lemon into a medium bowl. Dice the avocado and add to the bowl, tossing gently with your fingers to coat the pieces in lemon juice. Add the black beans, corn, and cilantro (if using). Toss gently until everything is thoroughly mixed.
Taste and add a pinch of salt if needed (if you've cooked the beans yourself, you'll probably want a small pinch; if you're using canned, chances are good that the already-salted beans will provide enough for the dish). Serve soon so that the avocado does not get a chance to darken.