put a value into hashmap which uses wildcard in java can not assign value

Question

I am studying Generics in java. I made a Map and initialized with 'new HashMap>()'. After that, I made an ArrayList to put into that variable. However, this code can not compile.

error message is:

"The method put(String, capture#1-of ? extends List) in the type Map> is not applicable for the arguments (String, List)"

on Map.put() method.

My code is below.

package org.owls.generic.main;

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

public class Main {
    public static void main(String[] args) {
        ArrayList<String> tmpArr = new ArrayList<String>();
        tmpArr.add("A");
        tmpArr.add("B");
        List<?> value = new ArrayList<String>(tmpArr);
        Map<String, ? extends List<String>> testMap = new HashMap<String, List<String>>();
        testMap.put("K", value);
    }
};

I can not understand why compiler looks type, even I assigned inheritanced Class which is 'List'.

Sorry for my poor English and edit will be welcomed. Thanks for your answer:D

Is there a reason you used wildcards? Are you experimenting with wildcards, or are you just trying to make it work? — Bohemian
– Bohemian ♦, Commented Dec 13, 2013 at 2:40
@OliCharlesworth So how can I put some type of Collection into Map<String, ?>? After I saw your comment, I changed List<?> value to ArrayList<String> value but still can not put it into Map. Is there any ideas?:) — Juneyoung Oh
– Juneyoung Oh, Commented Dec 13, 2013 at 7:35
@JuneyoungOh: You cannot put any value except null into a Map<String, ?> — newacct
– newacct, Commented Dec 16, 2013 at 10:25

newacct · Accepted Answer · 2013-12-16 10:24:26Z

3

You cannot put anything except null into a generic type with the ? extends ... parameter. This is basic Generics -- since the type parameter is unknown, you cannot put anything into it because whatever you try to put is not guaranteed to be a subtype of that unknown type.

Recall the PECS (Producer extends, Consumer super) rule. ? extends can only be used for producers. Putting something in is a Consumer.

answered Dec 16, 2013 at 10:24

newacct

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1 Comment

Juneyoung Oh Over a year ago

Thanks. Now I realize that I have tried something impossible:D

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