level curves in java

Hi,

I have to write a program which can draw contour plot(level curves) of every function, but i do not know how to calculate x and y in f(x,y)=z to draw the level curves?
Actually my program receives step size, first level and number of contours from user then it has to calculate the collection of all points (x,y) in the domain of a function f for which f(x,y) = c .
then it will show all level curves in related area
I want to have some result similar to this site:

http://mathdl.maa.org/images/upload_library/47/Seeburger/CalcPlot3D.htm

but I do not know how to find the exact value of x and y when f(x,y)= c ?

Thanks in advance