Forex, the speculative market acts the same as the futures market. Yes, it is true that you can actually aquire physical currency, but that’s not what you’re doing when you trade spot forex. Technically, spot forex trading operates the same as futures, though on a very short duration basis. If you buy USD/JPY you are technically making an agreement to exchange JPY for USD in two business day’s time at an agreed upon price. Of course you won’t actually do that, just like most futures traders will never take or make delivery on the positions they run. That’s where the rollover comes in to forex trading. Your broker essentially offsets your open position, closing it out, and enters a new position - all done at the current market rate. In some brokers this is very clearly done, especially when you’re talking about interbank trading. 

Most retail brokers don’t really work that way, though. Instead, when you buy USD/JPY you actually borrow JPY, convert it to USD and deposit the USD, with the guy on the other side of the trade doing the inverse. That’s where the interest rate differential comes in. Again, you have a situation where any gain you make is a loss for the other guy, and vice versa because you both have a requirement to repay the respective loans you have taken out. It’s not as if you could actually take the USD you bought and walk away.