A DESCRIPTION OF THE PROBLEM :
toMap(Functions.identity(),

There is no "Functions.identity()," plural.
the correct would be "Function.identity()" singular!

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Function.identity
ACTUAL -
Functions.identity

URL OF FAULTY DOCUMENTATION :
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#toMap-java.util.function.Function-java.util.function.Function-