Sampling Methods

What if I break the class up into six (6) groups, then I survey two (2) random people from each of the groups?

What if I break the class up into six (6) groups, then I survey everyone from the group closest to me?

What if I randomly choose students from the entire class using a spinner wheel with everyone's name on it?

What if I choose every 4th student who enters the room?

What do you think it is called when I just use people who want to be a part of my study? (They really just self-select for me)