Question

I have a method that returns an instance of

Map<String, List<Foo>> x();

and another method that returns an instance of

Map<String, Collection<Foo>> y();

Now if I want to dynamically add one of this Maps in my field, how can I write the generics for it to work?

ie:

public class Bar {
    private Map<String, ? extends Collection<Foo>> myMap;

    public void initializer() {
       if(notImportant) myMap = x(); //OK
       else myMap = y(); // !OK (Need cast to (Map<String, ? extends Collection<Foo>>)
    } 
}

Now is it ok that I cast to the signature even though the y() is declared as being Collection?

If it is not ok to cast, can I somehow write this (Collection OR List)
I mean, List is a Collection, so it should somehow be possible.

private Map<String, Collection<Foo> | List<Foo>>> myMap;

Answer 1

I’m not sure what your problem is. This code (essentially your code) compiles just fine.

import java.util.*;
public class Generic {
    static class Foo {};
    static Map<String, List<Foo>> x() {
        return null;
    }
    static Map<String, Collection<Foo>> y() {
        return null;
    }
    static Map<String, ? extends Collection<Foo>> myMap;
    public static void main(String[] args) {
        myMap = x();
        myMap = y();
        myMap = new HashMap<String,SortedSet<Foo>>();
        for (Collection<Foo> value : myMap.values());
    }
}

You can NOT, however, do something like List<Integer|String>. Java generics type bounds just doesn’t work like that.