Is it possible to return raw object from generic method?

Question

research this method from Colletions class:

@SuppressWarnings("unchecked")
    public static final <T> Set<T> emptySet() {
        return (Set<T>) EMPTY_SET;
    }

Collections.emptySet() return `Set<Object>`

This behaviour differ from generic classes. If I instantiate generic class without <T> I work with raw object.

I noticed this after method accept generic set was failed as compilation error:

public class GenericMethodRaw {
    public static void main(String [] args){
        Set set = Collections.emptySet();
        method(set); //fine
        method(Collections.emptySet());  //compilation error here
        Set<String> set1 = Collections.emptySet();//fine compilation
    }
    public static void method(Set<String> strings){}
}

compiler message:

java: method method in class GenericMethodRaw cannot be applied to given types;
  required: java.util.Set<java.lang.String>
  found: java.util.Set<java.lang.Object>
  reason: actual argument java.util.Set<java.lang.Object> cannot be converted to java.util.Set<java.lang.String> by method invocation conversion

Your last sentence makes no sense to me, would you mind rephrasing? Also; I see a question in the title, but none in the actual question text. Normally the title should summarize the question, and you should include a question in the body. Altough that might just be me.. — Patrick, Aug 08 '14 at 14:51
If I understand correctly, the problem you mention is a compilation error because the result of a method like "Collections.emptySet" was not usable in a method that accepts a raw collection? Could you provide an example? — bourbert, Aug 08 '14 at 14:54
My method argument has type Set . I expected that method return raw type and this value will acceptable for my method — gstackoverflow, Aug 08 '14 at 15:00
@gstackoverflow you should do `yourMethod(Collections.emptySet())` — Kent, Aug 08 '14 at 15:03
Your sample compiles in Java 8 which has better type inference. — Sotirios Delimanolis, Aug 09 '14 at 17:00

Sotirios Delimanolis · Accepted Answer · 2014-08-08T15:15:40.570

1

Terminology wise, an object isn't raw. The type of a reference value can be. The type of the value returned by a method must be compatible with the method's return type. In the emptySet method it is because of the cast.

In any case, the client will never see what happens in the call, all they know is that the method invocation expression has a type based on its invocation context.

edited Aug 08 '14 at 15:15

answered Aug 08 '14 at 14:52

Sotirios Delimanolis

274,122
60
696
724

Types are [raw](http://docs.oracle.com/javase/tutorial/java/generics/rawTypes.html). – Mike Samuel Aug 08 '14 at 15:00
Can I supress type evaluation in generic method? – gstackoverflow Aug 09 '14 at 08:22
@gstackoverflow Can you clarify what you mean by that? – Sotirios Delimanolis Aug 09 '14 at 16:57
I have generic class. For example ArrayList. If I create it follows ArrayList a = new ArrayList() - I can work as with raw type. At this case I can forgot about generic. – gstackoverflow Aug 09 '14 at 18:19
@gstackoverflow Within the generic method, you cannot suppress the method's generic type parameter, if that's what you're asking. – Sotirios Delimanolis Aug 09 '14 at 18:23

score 0 · Answer 2 · answered Aug 08 '14 at 15:02

0

If you really need to create a set with raw object, consider creating it with wildcard.

Set<?> set = Collections.emptySet();

answered Aug 08 '14 at 15:02

AnkitSomani

1,152
8
9

score 0 · Answer 3 · answered Aug 09 '14 at 08:19

0

Yes it is possible in concrete example:

Collections class has public field for this:

Collections.EMPTY_SET

In general looks like you cannot use raw version of generic method. Compiler would try evaluate type anyway.

answered Aug 09 '14 at 08:19

gstackoverflow

36,709
117
359
710

Is it possible to return raw object from generic method?

3 Answers3