How to "cast" HashMap to HashMap

Question

I have got this situation:

public abstract class Parent {
    public Parent(){}
}

public class Child extends Parent{
    public Child(){
    }
}

public class Main {
    public static void main(String[] args) {
        HashMap<Child, Double> mapChild = new HashMap<>();
        HashMap<Parent, Double> mapParent = new HashMap<>();

        foo(mapChild, new Child()); //Wrong 1 arg type
        foo(mapParent, new Child());
    }

    public static void foo(HashMap<Parent, Double> x, Parent parent){
        x.put(parent, 5.0);
    }
}

This code does not work, because foo(mapChild, new Child()) said - "Wrong argument type".
I tried somethig with Wildcards, but i think it cant work with it. I can create second foo method, but i do not want to duplicate code.

Any ideas?

PECS. https://stackoverflow.com/a/2723538/342852 – Sean Patrick Floyd Nov 01 '17 at 20:51 — Sean Patrick Floyd, Nov 01 '17 at 20:51

score 1 · Answer 1 · answered Nov 01 '17 at 20:51

1

Use

<? extends Parent>

in your collection. So the collection can accept both Child and Parent.

answered Nov 01 '17 at 20:51

Xavier Bouclet

922
2
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That is actually the wrong approach. It would be better to make the method `foo(...)` generic in the second parameter (`parent`) and set the `Map` to ` super T>` (where `T` is the type of the second argument). – Turing85 Nov 01 '17 at 20:58
He said any idea. It was mine. Just so you know your comment would have been much better without the first sentence. We are all here sharing ideas. With different level and skills. I didn’t think about the accepted solution. I am glad other did. – Xavier Bouclet Nov 02 '17 at 20:28

score 1 · Accepted Answer · answered Nov 01 '17 at 20:53

1

I believe what you want is

public static <T> void foo(Map<T, Double> x, T t) {
  x.put(t, 5.0);
}

...not to actually put a Parent object in a Map<Child, Double>.

answered Nov 01 '17 at 20:53

Louis Wasserman

191,574
25
345
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1

even better would be `Map super T, Double>`. – Turing85 Nov 01 '17 at 20:58
2

@Turing85: nah. They're equivalent, for this use case; `T` will get lifted to the appropriate type. – Louis Wasserman Nov 01 '17 at 20:59
true that. Took me a while to convince myself, but it seems that `extends` and `super` to a generic parameter is superfluous. – Turing85 Nov 01 '17 at 21:08
1

Would it be correct / useful to have T `extends Parent` like this: `public static void foo(...)`? – Alexandre Dupriez Nov 01 '17 at 22:13
You _could,_ but why bother adding the constraint when none of the logic actually cares? – Louis Wasserman Nov 01 '17 at 23:07
This is exactly what i needed. I used @AlexandreDupriez version, because my method needs it. Thank you guys ;) – B_Osipiuk Nov 01 '17 at 23:58

How to "cast" HashMap to HashMap

2 Answers2