So you wanna play with the exemplar of Demancia. Great!! you are making a good choice! Jarvan exels in the battlefiels as a great off tank and a great pure tank. You may be asking why not pure AD?. I promise to explain why not to do it.
Yes i know... everybody wanna get kills, everybody likes to do a pentakill and rape the enemies until they Rage and quit. Well i gonna say this: Altought Jarvan IV has a great damage output all his abilities are focused in a job that i say it's the Hardest job in League of Legends: that job is

INITIATE

. A few capters more ahead i'll explain how to do that and then you will see why NOT to go FULL AD.
I also opened to suggestions, so if you got something to correct in this guide or an idea to improve, please don't troll. Help me to make it better and don't just comment "this guide sucks!!"

Why you should play Jarvan IV

-Great Initiator
-Decent Farmer
-Good jungler
-Isn't too supceptible to ganks
-Nice choice to solo top
-He can works awesome with a good jungler.
-Is almost unkillable in late game
-He can work as tank and still be Letal

Some things you must consider:
-Initiate (your role) is one of the hardest roles in the game. That means two basic things: 1) your job is isolate the dangerous member of the enemy. If you fail your AP carry die, then your AD carry, then your support, and then... well you.

-YOUR JOB IS NOT TO KILL
-YOUR JOB IS NOT TO KILL
-YOUR JOB IS NOT TO KILL

-Your ultimate is easy to counter: not only with Flash. Theres a lot of champions abilities to scape from your cataclysm zone like Shunpo, Spinning Slash , Slice and Dice or Distortion for example.
-Another thing about Cataclysm: You can trap your squishies teamates and the enemy AD carry. That thing CANNOT/ NEVER CAN occur. Let me put more simple: if your Veigar and the enemy Tryndamere are into your ultimate zone means that is YOUR FAILURE!!

As you can see my masteries are focused in tanking functions wich is the function of this build with Jarvan IV. I think that masteries and runes are mostly beneficts from the early game. Not all but yes someones. I really don't want to keep you here reading the obvious. a 0-21-9 is not exactly an agro choice, specialy with our beloved Exemplar of Demancia

Remember that i say "runes and masteries are an early-game advantage some times"?
Well this is the case. I took armor penetration, Magic Resistance and Armor. Seals are optionals. I took Mana Regen just to keep the most posible mana to reach 6 and take an enemy in the fire range of my turret with Cataclysm. Anyways i tried to do an agressive and sustantiable character to hold the lane long as possible.
Quintessences are optionals too. I try to stablish a balance. The most arguable is the attac speed rune.

Passive
Martial Cadence
This a great passive, specialy for farming. Your firsts hits blows 8% of the target current life in magic damage. Mostly of the times Jarvan IV goes to solo top, so your passive is fantastic help to aplly the last hits on the creeps and take good amounts of gold.

Dragon Strike
It is a powerfull ability, makes a nice damage and reduce the armor. But the most important thing is his function combined with your Demacian Standard. You will get not only the prize of a good amount of damage. If your Dragon Strike touches your Demacian Standard our beloved Jarvan pass trough the enemies betwen you and your E and knockback's them. This is a Fantastic combo!!!! Not only is good to reach your enemies. Your E,Q combo allows you to scape trough walls which makes Jarvan IV a difficult champion to gank. It is great to initate too.

Golden Aegis
A nice abiity. Shields you and slow the enemies. As more enemy champions are close to you, more is the shield that you recieve. Another ability usefull to chase/scape. SWEET!

Demacian Standard
Not only works great with your Q. Your standard is very usefull due his passive (armor and attack speed) to you and your teamates once is trowed, giving his passive to them and you of course.

Cataclysm
Your ultimate. This is why Jarvan IV can be loved or hated for his team. Let me explain: Cataclysm is a "double edge sword". You can benefict your team by using on an enemy carry or blowit up by closing your Out of mana Malzahar (who's trying to scape) with the beatifull company of a Tryndamere. Whenever, and considering that this must not happen, you must try to use this skill to isolate the dangerous member of the enemies.

1 Mercury's Treads or Ninja Tabi
These bots are the options to Jarvan IV due his role in the game. A Crowd Controled Jarvan is not usefull at all to his team. The ninja's are the second choice against a heavy AD team. I don't wanna waste time to read on the bots, so NEXT!!

2:

Randuin's Omen

A must to Jarvan. His position in the teamfights is the heat of the battle, knockbacing, slowing and prisioning the enemies onto his ultimate. Due that you will use your R over a heavy dps the active of this item is awsome: Slow movement and attack speed in a decent area. also gives armor, health and HP regen. Anything else??

3:

Banshee's Veil

Our beloved Prince needs an item to avoid the CC of their rivals. Also is great the extra Mana and HP besides the MR and the passive. Of course you can choose a Force of Nature or a Quicksilver Sash both are great choices. FON will give you more MR, movement speed and a cool HP regen. QS it's good to scape the CC efect that YOU choose

4:

Thornmail

More armor yes! but you will get too the 30% of damage returned. Great against squishes AD like Ashe or Teemo for example. Once u get this item you are dangerous and the other team will think twice the idea of a gank over you. Anyways: thornamail it's a sitational item if the other team has at least 2 dangerous AD's. Will help you with Vayne but not too much against an Irelia. I gonna put it more simple: if the other team comp has no AD carry source this item is useless and you don't need it and you are wasting a slot.

5:

Warmog's Armor

Life, that's it. If you are looking this guide with good eyes and you wanna follow it, you need to understand that Jarvan IV needs to survive as long as possible. You will jump to the whole enemy team, making them to focus you and giving the time that your team need's to rape those bastards. This item will give you life and it's not so hard to farm it because you will get many assistances and your farming in late game is cool.

6:

Trinity Force

This is your offensive item. Gives you a great damage output, slow, movement speed, critical chance.... well. Due that you are a baddass who is very hard to take down you are now dangerous to the squishes. Will increase the damage of your abilities, will increase your first hits scaling with your passive (i'm talking about you Sheen) will increase your attack speed to give chances to slow the enemies due the Phage. And yes! will give you more movement speed and a bit of life wich is great.

Alternatives:

Atma's Impaler: good but the percentage of life that scales with your ad is too low.

Madred's Bloodrazor: great item to pulverize the armor of the enemy tanks.

Infinity Edge: this baby will increase your damage and critical chances more than anything. Remember that your abilities -except your E- scales with AD

As i promise i will explain why an AD Jarvan is not the best choice.

Think about your abilities. Did you notice that all are focused in the initiaton role?? I mean: a E-Q combo to go trough the enemies and knockbacking, an ultimate to jump over the enemy and close him with you. A shield which increases his beneficts over the amount of enemy champions near you....
I mean "hello!!! don't you see??? Your place is on the heat of the battle, absorbing damage, CC'S, isolating the enemies of your teamates: your Dragon Strike prupose is blow the armor of the enemies to give the chance to your ad carry to rape in those seconds.
A pure AD will not longer more than 2 seconds until that alistair knocs you back and the rest of his team will only need another CC to **** you up before you touch the ground!!

Your role is to initiate, you have to ruin the enemy game and give the time your carry's to kill them. Believe me: a 17-6-12 Jarvan it's not so valuable as a 0-3-38 Jarvan. As long as i play this game i've understand the sacrifice that someone must afford. Yea, i recognize the fun that is get 28 kills, i know the satisfaction of a PENTAKILL and your name in the screen, it feel's sooo great it's marvelous.

But you know what? there's something more great than that and it's to see the VICTORYanounce at the end of the game

Main spells:
Flash: it explain for himself. Flash is most common spell in LoL. It is awesome for scape, diving, engaging, chase. Altought Jarvan has the Demacian Standard and Dragon Strike combo he's a little mana hungry and this sequence consumes a decent amount of it.

Ignite: great to make a nice damage. This baby is great to make kills on early game. In mid and late game will give you assistances wich should be the main source of your money entrance.

Good choices:
Ghost: alternative to Flash. If you are a skilled Jarvan player and you dominate completly the art of E-Q combo this is your spell. I find myself using too. It's a free and great choice depending of your game style.

Teleport: Very usefull spell, specialy if you go mid (a very unusual place to Jarvan, but i've saw it) or solo top to keep you in the lane.

Smite: Jarvan is a viable champion to jungle so take it if it is your role.

Exhaust: a nice choice too, you can replace it for ignite to screw the enemy AD or chase. Same as Ghost: feel free to pick this spell.

Cleanse: if your enemies got heavy CC this can save your life.

Fortify: if you are the only tanky on your team (wich is not the best) you may need to take it, specialy if the enemy got pushers.

Not good choices:
Clairvoyance: this shoulda be the spell of your support, not yours. Get wards!

Heal: NO! buy pots. this spell is a waste in late game and you don't suppose to die having a complete arsenal to scape.

Rally: just NO!

Clarity: you got runes to regenerate mana, you are not a caster and there's a lot of spells more usefull.

Revive: if i see someone taking this spell i know he/she gonna die a lot and mostly of times dies double.