Making sure player coordinate isn't in ground

So, I'm making a command which will teleport you to a random place within 0,0 and 5000, 5000. I've got the X and Z coordinates working, but whenever I run the command, the player is put in the ground or in the air. How can I find the Y coordinate directly above the ground for a certain x/z coordinate?

Code:

Player ttp = (Player) sender;
Random r = new Random();
Location loc = new Location(((Player) sender).getWorld(), r.nextInt(5000) + 1, ???, r.nextInt(5000)+1);
ttp.teleport(loc);

 

This is what you are looking for.
Also i hope you are checking if sender is a Player, and I also hope you realise that coordinates can also be negative

 

When picking the coordinates check if the block at that location is air or not. If not air loop through the Y value at that location and find a place where it is air, but make sure that its not just 1 block of air which could also suffocate you.