While implementing HashMap: type mismatch when trying to program to interface

Question

I am trying to implement my own version of HashMap. FDor storing the items I would like to use an ArrayList of LinkedLists and for that I am taking "program to interface" approach. So the items type is List <List<MyEntry<k,v>>>

but

I wanna instantiate it by ArrayList< LinkedList< MyEntry< k,v>>>()

what is wrong with my code that I get compilation error

Type mismatch: cannot convert from ArrayList< LinkedList< MyHashMap1< k,v>.MyEntry<k,v>>> to List<List<MyHashMap1<k,v>.MyEntry<k,v>>>

My question is

If I change the type to List< LinkedList< MyEntry< k, v>>> It works but List< LinkedList< MyEntry< k, v>>> is not %100 interface, LInkedList is not an interface

package maps;
import java.util.ArrayList;
import java.util.List;
import java.util.LinkedList;
public class MyHashMap1<k,v> {
    //Type mismatch: cannot convert from ArrayList<LinkedList<MyHashMap1<k,v>.MyEntry<k,v>>> to List<List<MyHashMap1<k,v>.MyEntry<k,v>>>
    List <List<MyEntry<k,v>>> items = new ArrayList<LinkedList<MyEntry<k,v>>>(); 
    public class MyEntry<a,b>{
        a key;
        b value;

        public MyEntry(a key,b value){
            this.key = key;
            this.value = value;
        }
    }
}

Also see http://stackoverflow.com/questions/12861726/why-cant-you-have-a-list-of-lists-in-java — Alexis C., May 08 '14 at 18:13
That's not the issue. Teh issue is that If I change the type of items to List>> it works. but is List>> an interface? — C graphics, May 08 '14 at 18:18
Guys I updated my question ( added import java.util.LinkedList; ) sorry — C graphics, May 08 '14 at 18:25

Braj · Accepted Answer · 2014-05-08T18:25:57.813

1

Can you try this one

List<LinkedList<MyEntry<k,v>>> items = new ArrayList<LinkedList<MyEntry<k,v>>>();

or try this one if ArrayList can accept any type of List

List<List<MyEntry<k,v>>> items = new ArrayList<List<MyEntry<k,v>>>();

Always code to interface rather than concrete class.

edited May 08 '14 at 18:25

answered May 08 '14 at 18:16

Braj

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About 1st solution: is List>> an interface? – C graphics May 08 '14 at 18:18
`List` is an interface. – Braj May 08 '14 at 18:19
I meant the whole thing (List>> ) is an interface? – C graphics May 08 '14 at 18:20
`List>>` is 100% an interface if `MyEntry` is also an interface. – Braj May 08 '14 at 18:21
type information is lost at run-time hence there is no meaning of how you are defining the type information. here `List` is `interface` no matter how it is parametrized. – Braj May 08 '14 at 18:23
yes `List>>` is an interface. – Braj May 08 '14 at 18:24
@Cgraphics what about my second option? – Braj May 08 '14 at 18:28

While implementing HashMap: type mismatch when trying to program to interface

1 Answers1