You can’t nest method references. You can use lambda expressions instead:
return Comparator
.comparing(l->l.getCourse().getTeacher().getAge(), Comparator.reverseOrder())
.thenComparing(l->l.getCourse().getStudentSize());
Without the need for reverse order it’s even less verbose:
return Comparator
.comparing(l->l.getCourse().getTeacher().getAge())
.thenComparing(l->l.getCourse().getStudentSize());
Note: in some cases you need to explicitly state the generic types. For example, the code below won’t work without the <FlightAssignment, LocalDateTime>
before comparing(...)
in Java 8.
flightAssignmentList.sort(Comparator
.<FlightAssignment, LocalDateTime>comparing(a -> a.getFlight().getDepartureUTCDateTime())
.thenComparing(a -> a.getFlight().getArrivalUTCDateTime())
.thenComparing(FlightAssignment::getId));
Newer java version have better auto type detection and might not require that.