Java泛型类型推断

Question

I am trying to pass

Map<String, Map<String, List<TranslationImport>>> variable

to super class's constructor that expects:

Map<String, Map<String, List>>  

I tried changing parent class's constructor to expect

Map<String, Map<String, List<?>>> 

and

Map<String, Map<String, ? extends List>> 

to no avail.

Answer 1

If I understand correctly, you can change the superclass constructor signature; you just need something that accepts an arbitrary value for the List element type?

That should probably be Map<String, ? extends Map<String, ? extends List<?>>> Map<String, ? extends Map<String, ? extends List<?>>> Map<String, ? extends Map<String, ? extends List<?>>> .

Answer 2

Maybe this will "help" a litle

static void test(Map<String, Map<String, List>> m) {
    System.out.println(m);
}
public static void main(String[] args) {
    Map<String, Map<String, List<TranslationImport>>> variable = new HashMap<String, Map<String, List<TranslationImport>>>();
    Map<String, Map<String, List>> m2=(Map)variable;
    test(m2);
}

Answer 3

the problem is that capture conversion is not applied recursively:

http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.10

so '?' within nested < > is ignored for capture conversion

one way (although not completely satisfactory from the point of view of getting any value from generics) to make your code work would be to do something like:

import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;

public class Foo extends FooParent {
    Foo(Map<String, Map<String, List<TranslationImport>>> variable) {
        super(variable);
        System.out.println("ok");
    }
    public static void main(String[] args) {
        Map<String, Map<String, List<TranslationImport>>> var =
            new HashMap<String, Map<String, List<TranslationImport>>>();
        Foo foo = new Foo(var);
    }
}

class FooParent {
    FooParent(Map<String, ?> variable) {
        System.out.println("FooParent constructor");
    }
}

class TranslationImport {
}