Question

I am trying to write a generic function which will accept both of the following data types

Map <Integer, Map<Integer, Long>>
Map <Integer, Map<Integer, Double>>

My function looks like this,

function(Map<Integer, Map<Integer, ? extends Number>> arg) {}

But I am getting an incompatible type error. It works for a Map, but not for map of Maps. I am not able to understand why? Is there any way to do this?

Answer 1

First let’s reduce the problem by using Sets instead:

Set<Set<Long>> longSetSet = null;
Set<Set<Double>> doubleSetSet = null;

Set<Set<? extends Number>> someNumberSetSet;

// try assigning them
someNumberSetSet = longSetSet;   //
someNumberSetSet = doubleSetSet; // compiler errors - incompatible types

At first glance you might wonder why this assignment is illegal, since after all you can assign a Set<Long> to Set<? extends Number> The reason is that generics are not covariant. The compiler prevents you from assigning a Set<Set<Long>> to Set<Set<? extends Number>> for the same reason it won’t let you assign a Set<Long> to a Set<Number>. See the linked answer for more details.

As a workaround, you can use a type parameter in your method signature as other answers have suggested. You can also use another wildcard to make the assignment legal:

Set<? extends Set<? extends Number>> someNumberSetSet;

someNumberSetSet = longSetSet;   //
someNumberSetSet = doubleSetSet; // legal now

Or in your example:

function(Map<Integer, ? extends Map<Integer, ? extends Number>> arg) { }