Insert object into List wildcard extending interface

Question

So I have an interface

public interface Identity {

abstract void setKey(String key);
abstract String getKey();
}

That I want to use for an insert sort function

public static void InsertIdentity(Identity identity, ArrayList<? extends Identity> list) {
    int i = 0;
    Identity id;
    while (i < list.size()) {
        if ((id = list.get(i)).getKey().compareTo(identity.getKey()) < 0) {
            i++;
        }else{break;}
    }
    list.add(i, identity);
}

But obviously I cannot add an Identity to a ArrayList - ? extends Identity

I was wondering if there is a way around this?

I know Java has other ways of doing this task but I would like this function to work.

Possible duplicate of [Java generic collection, cannot add list to list](http://stackoverflow.com/questions/16305164/java-generic-collection-cannot-add-list-to-list). And there are many more questions dealing with this issue ... — Seelenvirtuose, Apr 23 '15 at 08:50
I know why it doesn't work, I was hoping someone could help me find a way around it — TheOnlyWAR10CK93, Apr 23 '15 at 08:50
The way around it is to not declare your parameter with an upper bounded wildcard. — Seelenvirtuose, Apr 23 '15 at 08:52
You obviously want to make your method work on _various types_. This cries for a type parameter: ` void insertIdentity(T identitiy, List list) { ... }`. By the way: Keep on using Java's code conventions, and use interfaces where appropriate. — Seelenvirtuose, Apr 23 '15 at 08:56

fabian · Accepted Answer · 2015-04-23T08:57:20.197

4

You could make the method generic:

public static <T extends Identity> void InsertIdentity(T identity, ArrayList<T> list) {
    //...

You may need to make other methods generic as well, if you use this approach.

edited Apr 23 '15 at 08:57

answered Apr 23 '15 at 08:54

fabian

80,457
12
86
114

Thank you very much! I was trying to do something like this earlier but couldn't get it to work. Now I know! – TheOnlyWAR10CK93 Apr 23 '15 at 08:56

score 3 · Answer 2 · answered Apr 23 '15 at 08:54

3

I would suggest the following:

public static <I extends Identity> void InsertIdentity(I identity, ArrayList<I> list)

answered Apr 23 '15 at 08:54

Blip

3,061
5
22
50

Thank you very much! I was trying to do something like this earlier but couldn't get it to work. Now I know! – TheOnlyWAR10CK93 Apr 23 '15 at 08:56

score 2 · Answer 3 · answered Apr 23 '15 at 08:55

2

This can work, depending on how you will use the InsertIdentity method:

public static <T extends Identity> void InsertIdentity(T identity, ArrayList<T> list) {

answered Apr 23 '15 at 08:55

Ramón Gil Moreno

809
5
19

Insert object into List wildcard extending interface

3 Answers3