>Is there another tutorial that can explain the fundamental of "emulator" first before the NES part kicks in?

>PS: I'm planning to use Java for this side-project.

One place to start might be to try and write an emulator for the DCPU-16. It's a simple made-up computer that Notch (creator of Minecraft) designed for the game 0x10c.

Although the game was never finished, many people wrote emulators based on the DCPU's specifications[1]; search Github for "DCPU Java" to see several examples in that language.

[1] https://raw.githubusercontent.com/gatesphere/demi-16/master/...