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Double post, but to answer your first question, my preffered build order is generally Wriggle's > Mobility boots > Locket > Brutalizer > Phage. From there I build for whatever I think I need. Locket and Brutalizer are huge for Jarvan because of their cooldown reduction, which is something Jarvan really wants so he can E>Q more often. Phage gives the chance of a slow proc, making sticking to targets potentially much easier. With phage, you're also free to decide between building Tri-Force if you're on a roll or Frozen Mallet if you need to be tankier. It's all a toss up after that. You can get stuff like Maw, Bloodthirster, Black Cleaver, and/or Scimitar if you think you can survive while building only damage; or you can get items like Bulwark, Warmog's, Atma's, and Randuin's Omen if you need to be tankier. You can also feel free to replace your mobility boots with some defensive boots later on in the game, where roaming isn't needed as much. I prefer the captain's enchantment for my boots since I generally am the initiatotr. Let's your teammates (hopefully) get to whoever you managed to trap with your ult and kill them faster, so you don't tank for as long.