How to pass generics using wildcard to a function without casting?

Question

Here is my problem: my function in class A:

public void setData(Map<String,? extends ArrayList<? extends SomeInterface>>){...}

my call:

Map<String, ArrayList<ImplementsSomeInterface>> a=...;
instanceOfA.setData(a); //does not compile
instanceOfA.setData((Map<String,? extends ArrayList<? extends SomeInterface>>) a); // works thanks to the Casting.

I don't think this is clean. Is there a way to avoid the casting without droping the wildcard use in the function?

Is class A generic? What does class A declaration look like? — claesv, Apr 26 '12 at 13:36
What do you mean it works? No ClassCastException when running it? What is subClassOfSomeClass? — assylias, Apr 26 '12 at 13:36
If `subClassOfSomeClass` is not an ArrayList of anything, how can this work? — sinelaw, Apr 26 '12 at 13:37
... and `subClassOfSomeClass` is or isn't a subclass of `ArrayList`? Because that's your type bound. — Marko Topolnik, Apr 26 '12 at 13:37
ooops sorry, I didn't see your comments while I was updating my question! — giscope, Apr 26 '12 at 13:40
@claesv A is just a normal Class, I don't think it affects the rest of the code — giscope, Apr 26 '12 at 13:41
@assylias no it works as though I wasn't using the wildcards — giscope, Apr 26 '12 at 13:42
you guys are too fast! It'll teach me to fully check my questions in the future. Thank you!! — giscope, Apr 26 '12 at 13:48

Nick Holt · Accepted Answer · 2012-04-26T14:10:38.427

3

First your setData method should read:

public void setData(Map<String,? extends List<? extends SomeInterface>>)

Your map declaration should read:

Map<String, List<ImplementsSomeInterface>> a = ...;

Which is pretty much what you've got following your edits to the original question. The change I've made from ArrayList to List doesn't effect the behaviour of the code.

Following comments:

public static void main() 
{
    Map<String, List<Double>> map = new HashMap<String, List<Double>>();
    map.put("prices", new ArrayList<Double>(Arrays.asList(1.1, 2.2, 3.3)));

    setData(map);
}

public static void setData(Map<String,? extends List<? extends Serializable>> map)      
{
}

edited Apr 26 '12 at 14:10

answered Apr 26 '12 at 13:44

Nick Holt

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assuming I use `public void setData(Map>)` I still have to cast the parameter in the call as follow `instanceOfA.setData((Map>) a);` is there a way to avoid the casting? – giscope Apr 26 '12 at 13:52
Not in the version of Java 1.6 I'm using, I tried it :-) I'll add the code to my answer. – Nick Holt Apr 26 '12 at 14:06

score 1 · Answer 2 · answered Apr 26 '12 at 13:43

The problem is that setData takes as an argument a Map<String,? extends ArrayList<? extends SomeClass>>Map> whereas in your first call(the one that doesn't compile) you are trying to pass it a Map<String,? extends SomeClass>

Java cannot automatically cast a subclass of SomeClass to a subclass of ArrayList<? extends SomeClass> because it is not neccesarily an instance of ArrayList.

How to pass generics using wildcard to a function without casting?

2 Answers2