Alkyl fluorides can be prepared by action of mercurous fluoride or antimony trifluorides (inorganic fluorides) on corresponding alkyl halide. This reaction is known as Swarts reaction.
$CH _{3} Br + AgF \longrightarrow \underset{\text{Methyl fluoride}}{CH _{3} F }+ AgBr$
But, when aciton of $Nal /$ acetone takes place on alkyl chloride of bromide, alkyl iodide forms. This reaction is called 'Finkelstein reaction'
$C _{2} H _{5} Cl \xrightarrow[\text{acetone}]{Nal} C _{2} H _{5} I + NaCl$
Free redical fluorination is highly explosive reaction, so not preferred for the preparation of fluoride.