Can exes be friends or not?

Given the fact that my ex boyfriend said he wouldn't be friends anymore if I found a new boyfriend. But I am not thinking of dating anyone until I graduate Uni because I spent too much time on him before and it feels like I didn't get anything else done and the relationship ended so it's not worth my time at the moment. Will a friendship work out? He did say if we got to really good friends he wouldn't leave me if he got a new girlfriend type thing
Can exes be friends or not?
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