question: @ Nullable public static HKey psiKey ( @ NotNull PsiMethod psiMethod , @ NotNull Direction direction , @ NotNull MessageDigest md ) { final PsiClass psiClass = PsiTreeUtil . getParentOfType ( psiMethod , PsiClass . class , false ) ; if ( psiClass == null ) { return null ; } byte [ ] classDigest = psiClassDigest ( psiClass , md ) ; if ( classDigest == null ) { return null ; } byte [ ] sigDigest = methodDigest ( psiMethod , md ) ; if ( sigDigest == null ) { return null ; } byte [ ] digest = new byte [ HASH_SIZE ] ; System . arraycopy ( classDigest , 0 , digest , 0 , CLASS_HASH_SIZE ) ; System . arraycopy ( sigDigest , 0 , digest , CLASS_HASH_SIZE , SIGNATURE_HASH_SIZE ) ; return new HKey ( digest , mkDirectionKey ( direction ) , true ) ; }
target:  Converts a Psi method to a small hash key (HKey). Returns null if conversion is impossible (something is not resolvable).