Question

I have a piece of code that I think should compile, but it doesn’t. Here is the code:

public class Program {
public void myMethod(List<EnumMap<? extends MyInterface, String>> map)
{

}

public void caller()
{
    EnumMap<MyEnum, String> map = new EnumMap<MyEnum, String>(MyEnum.class);

    List<EnumMap<MyEnum, String>> list = new LinkedList<EnumMap<MyEnum, String>>();

    myMethod(list); //error argument type is not compatible 

}

}

MyEnum is an enum that implements MyInterface.

Why does calling myMethod(list) gives me argument type is not compatible?

If I change myMethod’s signature to:

public void myMethod(List<? extends Map<? extends MyInterface, String>> map)

then everything works fine, but I’m still puzzled and wish to know why the original
method signature does not work.

Answer 1

EnumMap<MyEnum, String> and EnumMap<? extends MyInterface, String> are different types (the former is a subtype of the latter). So List<EnumMap<MyEnum, String>> and List<EnumMap<? extends MyInterface, String>> are not compatible with each other. In general, List<A> cannot be assigned to List<B> if A and B are different types, no matter what the relationship between A and B is (same applies to any generic type not just List). When you have a bounded type parameter, that is a different story; the compatibility takes the bounds into consideration.