Not understanding a Java polymorphism example involving HashMaps

Question

I'm rusty on my Java polymorphism.

If I have a class Merchandise, and then a class Clothing that extends Merchandise, why aren't I able to do the following?

HashMap<String, Merchandise> stuff = new HashMap<String, Clothing>();

When I do so, I am getting this compilation error:

DataStore.java:5: error: incompatible types: HashMap<String,Clothing> cannot be converted to HashMap<String,Merchandise>
        public static HashMap<String, Merchandise> tshirts = new HashMap<String, Clothing>();

Aren't all Clothing items also Merchandise items? ^

Possible duplicate of [Is List a subclass of List? Why aren't Java's generics implicitly polymorphic?](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p) — Savior, Apr 25 '16 at 05:03
Yeah, that link is very helpful, Pillar. I just wasn't searching on the right terms to hit it. Sorry for the duplication. — A B, Apr 25 '16 at 05:21

score 1 · Accepted Answer · answered Apr 25 '16 at 05:04

1

Consider the following scenario:

HashMap<String, Clothing> clothing = ...
HashMap<String, Merchandise> merchandise = clothing; // Suppose this is allowed
merchandise.put("Potato", new Potato());

Uh-oh, by putting potato into merchandise, it has also gone into clothing, because they reference the same object!

answered Apr 25 '16 at 05:04

Andrew Williamson

8,299
3
34
62

Thanks, Andrew. That make sense. – A B Apr 25 '16 at 05:21

score 1 · Answer 2 · answered Apr 25 '16 at 05:24

1

You need to do like this: HashMap<String,? extends merchandise> m=new HashMap<String,Clothing>();

HashMap<String, Merchandise> IS NOT a HashMap<String, Clothing>()>.

answered Apr 25 '16 at 05:24

ProblemSolver

161
1
1
10

Not understanding a Java polymorphism example involving HashMaps

2 Answers2