Error While Passing Bounded Wild Card Collection in Java

Question

For the below Class Structure:

class BaseClass{}
   class A extends BaseClass{}
   class B extends BaseClass{}

I have this method in main:

public static void goThroughMap(Map<String,Collection<? extends BaseClass>> myMap) {}

Now to call this method , i am doing something like:

Map<String,Collection<B>> myMap = new HashMap<>();
goThroughMap(myMap); // is causing a compile time error

The error is:

The method goThroughMap(Map<String,Collection<? extends BaseClass>>)  is not applicable for the arguments (Map<String,Collection<B>>)

So, how should i modify my method parameters to make this work? Any pointers for further reading are welcome.

I have seen related questions by other users(see links below), but none of them seem solve the issue here:

Please show how have you declared `B`. – AlexR Sep 02 '14 at 10:07 — AlexR, Sep 02 '14 at 10:07
What's the error you are getting? – Eypros Sep 02 '14 at 10:07 — Eypros, Sep 02 '14 at 10:07

score 0 · Answer 1 · answered Sep 02 '14 at 10:10

0

You have to generify your method properly:

public static <T extends BaseClass> void goThroughMap(Map<String, Collection<T>> myMap) {
  ...
}

answered Sep 02 '14 at 10:10

Andrew Logvinov

Can you point out what exactly is wrong with my declarations. Both your and Scrooge McD's answers work for me. – bartender Sep 02 '14 at 10:26

score 0 · Accepted Answer · answered Sep 02 '14 at 10:25

0

You can use the following declaration for your method :

public static void goThroughMap(Map<String,? extends Collection<? extends BaseClass>> myMap)

You can refer to this link for more details on such types : Generics FAQ

answered Sep 02 '14 at 10:25

Scrooge McD

This works for me. Going through the link to understand more about this. – bartender Sep 02 '14 at 10:29

2 Answers2