I guess you should be up to some point for this project. So it is better to mention wht kind of problem you are facing so that someone can help you up.

I guess  in Y=f(k)

First of all you have to find the Value of K  so everytime your vale changes  for K,the function F(k) change and your Y value  change too.

For simplicity if we say K is fixed... then i guess it is not a big deal to find the coordinate of Y.

once you have a value of Y AND K in java

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Graphics.html

you can use graphics to draw a require graph.

n = population growth rate
d = depreciation
k = capital per worker
y = output/income per worker
L = labor force
s = saving rate

Hope this will help you.