Jarvan IV Build
Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Every item in the build aims to complement the others, and to shore up one of Jarvan's weaknesses.
Regrowth Pendant is one of my favorite starting items for an off-tank, because it provides a tremendous amount of staying power and can build into a wide variety of solid defensive items later in the game. For this character, I build a Warmog's mid-late game.
Mercury's Treads and Aegis of the Legion combine with Demacian Standard to give Jarvan a pretty good amount of armor and magic resistance - a little over 100 each.
Manamune and Youmuu's both provide Jarvan with some much-needed damage, and also help to reduce or eliminate his problems with mana and some rather long cool downs. Dragon Strike spam is a must if you want to remain a threat.
Finally, for my sixth item I will usually pick up Atma's, but if the other team is very magic-dependent Wit's End is an excellent substitute.
The only parts of the build that I would consider unchangeable are Manamune, Youmuu's, and either Atma's or Wit's End. Feel free to drop the Treads for Ninja Tabi, or Aegis/Warmog's for Banshee's Veil or Randuin's.