Generic static method constrains types too much

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In this answer I attempted to create a static utility method to make a List into a Map:

public static <K, T> Map<K, T> toMapBy(List<T> list,
        Function<? super T, ? extends K> mapper) {
    return list.stream().collect(Collectors.toMap(mapper, Function.identity()));
}

It works just fine. However, I found that the method cannot be used in all the same contexts as the list.stream().collect(...) expression. The method isn't as flexible.

List<Student> students = Arrays.asList();

Map<Long, Student> studentsById1 = students.stream()
        .collect(Collectors.toMap(Student::getId, Function.identity()));
Map<Long, Student> studentsById2 = toMapBy(students, Student::getId);

Map<Long, Person> peopleById1 = students.stream()
        .collect(Collectors.toMap(Student::getId, Function.identity()));
Map<Long, Person> peopleById2 = toMapBy(students, Student::getId);  // compile error!

In this example, Student is a subtype of Person and has a getId method that returns a Long.

The last statement fails with incompatible types: inference variable T has incompatible bounds ... (JDK 1.8.0_25). Is there a way to define the type parameters so that the static method will work in the same contexts as the expression it contains?

asked 40 secs ago

glts

5,912

Generic static method constrains types too much