Add subclasses to ConcurrentHashMap of super type in a static initializer?

Question

public class People {

    class Family extends People {

    }

}


public class Together {
    private static Collection<Family> familyList = new ArrayList<Family>();
    private static ConcurrentMap<String, Collection<People>> registry = new ConcurrentHashMap<String, Collection<People>>();

    static {
        registry.put(Family.class.toString(), familyList); 
    }
}

Error message:

The method put(String, Collection<people>) in the type Map<String,Collection<people>> is not applicable for the arguments (String, Collection<family>)

Why can't I put familyList into registry? I figure that since family extendspeople that i should be able to place the sub types into the super type registry .

EDIT: The above is solved. My last part of the question involves a more complicated example using the same names:

public class Together {
    private static ConcurrentMap<String, Collection<Family>> familyMap= new ConcurrentHashMap<String, Collection<Family>>();
    private static ConcurrentMap<String, ConcurrentMap<String, Collection<People>>> registry2 = new ConcurrentHashMap<String, ConcurrentMap<String, Collection<People>>>();

    static {
        registry2.put(Family.class.toString(), familyMap); 
    }
}

(I already tried changing the declaration of registry2 to having ?extends People

Now the error is: The method put(String, ConcurrentMap<String,Collection<People>>) in the type Map<String,ConcurrentMap<String,Collection<People>>> is not applicable for the arguments (String, ConcurrentMap<String,Collection<Family>>)

http://stackoverflow.com/q/3763192/139010 and http://stackoverflow.com/a/2660850/139010 — Matt Ball, Jun 19 '13 at 20:30
The second problem is the same as the first. The solution should be: private static ConcurrentMap>> registry2 = new ConcurrentHashMap>>(); — axiopisty, Jun 19 '13 at 20:50
Hmm, doesn't work for me. I will post this in a new question. If you can answer it there, you could get the correct answer — Stephen D, Jun 19 '13 at 20:53

score 4 · Answer 1 · edited May 23 '17 at 11:50

4

Because a Collection<family> is not a Collection<people>. Put another way: Java collections are not covariant.

Is there a way I could put family into the hashmap?

Declare it as a Collection<people>.

edited May 23 '17 at 11:50

Community

1
1

answered Jun 19 '13 at 20:30

Matt Ball

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Is there a way I could put family into the hashmap? – Stephen D Jun 19 '13 at 20:31
1

@StephenD declare it as a `Collection`. – Matt Ball Jun 19 '13 at 20:31

score 3 · Answer 2 · answered Jun 19 '13 at 20:31

3

family is convertible to people, but Collection<family> is not convertible to Collection<people>.
Had it been convertible, you would have been able to unsafely add a different derived tyupe to the casted collection.

Instead, you can use a covariant view of the collection type:

ConcurrentMap<String, Collection<? extends people>>

answered Jun 19 '13 at 20:31

SLaks

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@StephenD: Which this? Safe against what? – SLaks Jun 20 '13 at 13:54
I'm not sure what I meant... haha – Stephen D Jun 20 '13 at 15:14

axiopisty · Accepted Answer · 2013-06-19T20:43:38.970

1

Try this:

people.java

public class people {

    public class family extends people {

    }

    public static void main(String[] args) {
        together t = new together();
        System.out.println(together.registry);
    }

}

together.java

import java.util.ArrayList;
import java.util.Collection;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

public class together {
    private static Collection<people.family> familyList = new ArrayList<people.family>();
    public static ConcurrentMap<String, Collection<? extends people>> registry = new ConcurrentHashMap<String, Collection<? extends people>>();

    static {
        registry.put(people.family.class.toString(), familyList);
    }

}

edited Jun 19 '13 at 20:43

answered Jun 19 '13 at 20:31

axiopisty

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Thanks for making this clear! I edited the question and will no doubt consult your answer for the first part of my question! – Stephen D Jun 19 '13 at 20:45
Great. I'm glad I could help. If this solution helped please select this as the correct answer. :) – axiopisty Jun 19 '13 at 20:47

Add subclasses to ConcurrentHashMap of super type in a static initializer?

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