Hi Kalash, 

Finkelstein reaction in which Halogen exchange takes place, prefered for the preparation of alkyl iodide (Iodoalkanes)

We can define it as,

"Alkyl Iodide prepared by the reaction of alkyl chloride or alkyl bromide with NaI in dry acetone. This reaction is knownn as Finkelstein reaction"

R-X + NaI --> R-I + NaX

Hope you got your answer

Thanks for asking