Why does this simple Java generic function not compile?

Question

While f1 does compile, the very similar f2 won't and I just cant explain why. (Tested on Intellij 9 and Eclipse 3.6)

And really I thought I was done with that kind of question.

import java.util.*;
public class Demo {

    public  List<? extends Set<Integer>> f1(){
        final List<HashSet<Integer>> list = null;
        return list;         
    } 

    public  List<List<? extends Set<Integer>>> f2(){
        final List<List<HashSet<Integer>>> list = null;
        return list;         
    } 

}

Answer 1

List<List<HashSet<Integer>>> is not assignable to List<List<? extends Set<Integer>>> for the same reason List<HashSet<Integer>> would not be assignable to List<Set<Integer>>.

You can get it to compile by changing this:

public  List<List<? extends Set<Integer>>> f2(){

into this:

public  List<? extends List<? extends Set<Integer>>> f2(){

The reason your code didn't compile, and why the other example I gave (ie: "List<HashSet<Integer>> would not be assignable to List<Set<Integer>>") is that Java generics are not covariant.

The canonical example is that even if Circle extends Shape, List<Circle> does not extend List<Shape>. If it did, then List<Circle> would need to have an add(Shape) method that accepts Square objects, but obviously you don't want to be able to add Square objects to a List<Circle>.

When you use a wildcard, you're getting a type that slices away certain methods. List<? extends Shape> retains the methods that return E, but it doesn't have any of the methods that take E as a parameter. This means you still have the E get(int) method, but add(E) is gone. List<? extends Shape> is a super-type of List<Shape> as well as List<Circle>, List<? extends Circle>, etc. (? super wildcards slice the other way: methods that return values of the type parameter are removed)

Your example is more complicated because it has nested type parameters, but it boils down to the same thing:

• List<HashSet<Integer>> is a sub-type of List<? extends Set<Integer>>
• Because generics are not covariant, wrapping the two types in a generic type (like List<...>) yields a pair of types that no longer have the sub/super-type relationship. That is, List<List<HashSet<Integer>>> is not a sub-type of List<List<? extends Set<Integer>>>
• If instead of wrapping with List<...> you wrap with List<? extends ...> you'll end up with the original relationship being preserved. (This is just a rule of thumb, but it probably covers 80% of the cases where you'd want to use wildcards.)

Note that trashgod and BalusC are both correct in that you probably don't want to be returning such a weird type. List<List<Set<Integer>>> would be a more normal return type to use. That should work fine as long as you're consistent about always using the collection interfaces rather than the concrete collection classes as type parameters. eg: you can't assign a List<ImmutableSet<Integer>> to a List<Set<Integer>>, but you can put ImmutableSet<Integer> instances into a List<Set<Integer>>, so never say List<ImmutableSet<Integer>>, say List<Set<Integer>>.

Answer 2

"Do not use wildcard types as return types. Rather than providing additional flexibility for your users, it would force them to use wildcard types in client code."—Joshua Bloch, Effective Java Second Edition, Chapter 5, Item 28.