A DESCRIPTION OF THE PROBLEM :
the example code :
     Map<Student, Double> studentToGPA
         students.stream().collect(toMap(Functions.identity(),
                                         student -> computeGPA(student)));

uses Functions.identity() while the interface name is Function.identity()

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
     Map<Student, Double> studentToGPA =
         students.stream().collect(Collectors.toMap(Function.identity(),
                                         student -> computeGPA(student)));
ACTUAL -
     Map<Student, Double> studentToGPA
         students.stream().collect(toMap(Functions.identity(),
                                         student -> computeGPA(student)));

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-