Well i want to save a variable that even after user quit the game it remain the same.So how could i make a file in which i could save it.

Have you searched?  this is well documented.  There is also good examples in the bukkit community that could be cross referenced.

 

After you have done all that, let us know what you had trouble with.

As always: It depends™.

Where does that variable belong to?

To a Block? Use a TileEntity or metadata (only 4 bits, most likely not enough if you say "variable").

To an Item? Use the NBTTagCompound attached to every ItemStack.

To an Entity? Use writeToNBT / readFromNBT or IExtendedEntityProperties in case it is not your entity.

To a specific world (=dimension)? Use WorldSavedData with World#perWorldStorage.

To a save file? Use WorldSavedData with World.mapStorage.

To the Mod in general? Use the Configuration class provided by forge, or just a normal file for more complex data (you can use the normal Java file writing mechanics here).

 

I want to do it on an food item.

Can you give an example on how can i do this.

Thanks i am able to add data to nbt but how can i take data from there.

I want to use the if statement for the data in TickHandler.

I dont know how can i check that. If that nbt key have the proper data saved in it.

To get the data from the ItemStack you need to get the ItemStack first of course. Where do you want that to be? In the players inventory? Somewhere else?

Well as it is a food item so i wanna check if player has ever eaten this which i am storing in the nbt then on tickhandler i wanna check that if the player has ever eaten that and if true then the process carry on if false than break.

 

Actually i am trying to make an infinite potioneffect for the food. That can last even after the player die or close the game or drink milk. I will add a new item which will break this process.

Then... the variable doesn't belong to the Item, because the Item is eaten. -.-

You need to store it into the Player, use IExtendedEntityProperties.

Can you give example on how to use that

I think this tutorial might be help:

http://www.minecraftforum.net/topic/1952901-172164-eventhandler-and-iextendedentityproperties/