Generic static method constrains types too much

Question

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

public static  Map toMa         


        

Answer 1

Your last line calls the method toMapBy in which the compiler infers the type Student for T. So it obviously returns a List.

But generics aren't covariant!

That means, you cannot assign a List to a variable of type List, because they are not in a subtype relationship.

The solution is to use the subtype form:

Map peopleById2 = toMapBy(students, Student::getId); // no compiler error