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Generic Arguments of type Enum -- Can't Call valueOf()

Posted on 2010-08-23
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Last Modified: 2012-05-10
In the code, the two lines where the method "valueOf()" is called on my generic parameters of type Enum are giving an error:

"The method valueOf(String) is undefined for the type class <capture#3 of ? extends Enum>"

But my generic argument is of type enum, so I don't understand this error.  

Ulitmately, my goal is to convert a string representation of a hashmap (whose keys are enums and whose values are arraylists of enums) into a proper hashmap of the correct enum types.  Since there are different possible enums that can be used as the keys and values, I need a generic solution to avoid recreating the same method over and over for all the types.  

I am also confused by the definition of the Enum type:

public abstract class Enum<E extends Enum<E>>

It appears that parameterizing it by itself would create an infinite regression.  

Thanks,
Jonah
protected static <K extends Enum<?>, V extends Enum<?>> HashMap<K,ArrayList<V>> getHmap(String s, K keyType, V valType) {
        HashMap<K, ArrayList<V>> ret = new HashMap<K, ArrayList<V>>();
        HashMap<String, String[]> strHashmap = getStringHashmap(s);
        Iterator<String> iter = strHashmap.keySet().iterator();
        while (iter.hasNext()) {
            String oldKey = iter.next();
            String[] oldVals = strHashmap.get(oldKey);
            K newKey = keyType.getClass().valueOf(oldKey);
            ArrayList<V> newVals = new ArrayList<V>(oldVals.length);
	        for(int i=0; i<oldVals.length; i++) {
	            newVals.add(valType.getClass().valueOf( oldVals[i]));
	        }
	        ret.put(newKey, newVals);
        }
        return ret;

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Question by:Jonah11
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21 Comments
 
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Expert Comment

by:objects
ID: 33508299
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Author Comment

by:Jonah11
ID: 33508323
What about usage like this:

http://www.java2s.com/Code/JavaAPI/java.lang/EnumvalueOfStringv.htm

Also, even if I change the code to:
K newKey = Enum.valueOf(keyType.getClass(),oldKey);

it still does not work.
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Expert Comment

by:CEHJ
ID: 33508430
Try
            K newKey = K.valueOf(keyType.getDeclaringClass(), oldKey);
            ArrayList<V> newVals = new ArrayList<V>(oldVals.length);

            for (int i = 0; i < oldVals.length; i++) {
                newVals.add(valType.valueOf(valType.getDeclaringClass(), oldVals[i]));
            }

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Accepted Solution

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CEHJ earned 250 total points
ID: 33508451
That needs the following method prototype (adjusted to return the more generic and flexible Map as opposed to HashMap)
protected static <K extends Enum<K>, V extends Enum<V>> Map<K, List<V>> getMap(
        String s, K keyType, V valType)

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Expert Comment

by:CEHJ
ID: 33508472
The whole method:
    protected static <K extends Enum<K>, V extends Enum<V>> Map<K, List<V>> getMap(
        String s, K keyType, V valType) {
        Map<K, List<V>> ret = null;

        HashMap<String, String[]> strHashmap = getStringHashmap(s);
        Iterator<String> iter = strHashmap.keySet().iterator();

        while (iter.hasNext()) {
            String oldKey = iter.next();
            String[] oldVals = strHashmap.get(oldKey);
            K newKey = K.valueOf(keyType.getDeclaringClass(), oldKey);
            List<V> newVals = new ArrayList<V>(oldVals.length);

            for (int i = 0; i < oldVals.length; i++) {
                newVals.add(valType.valueOf(valType.getDeclaringClass(), oldVals[i]));
            }

            ret.put(newKey, newVals);
        }

        return ret;
    }

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Author Comment

by:Jonah11
ID: 33512610
CEHJ,

Thanks very much, that works.  I still don't completely understand why, however.  Can you answer a few more questions:

1.  Why does getDeclaringClass work but getClass does not?  
2.  Why does the syntax used in this example NOT work:
http://www.java2s.com/Code/JavaAPI/java.lang/EnumvalueOfStringv.htm
3.  Why did we need to use the syntax "<K extends Enum<K>", which is also used by the definition of the Enum class itself?  As I said, it seems to be self referential and thus confuses me.

Thanks again,
Jonah
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Expert Comment

by:objects
ID: 33516340
Don't you love it when someone posts some code but cannot actually explain it :)
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Expert Comment

by:objects
ID: 33516394
Will see if I can help you

> 1.  Why does getDeclaringClass work but getClass does not?  

The two aren't actually related, you need Enum.getDeclaringClass() to the the Enum class to call valueOf() as I mentioned in my earlier comment

http://download.oracle.com/javase/6/docs/api/java/lang/Enum.html#getDeclaringClass()

> 2.  Why does the syntax used in this example NOT work:

thats a special case I think, will check that for you
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Expert Comment

by:CEHJ
ID: 33516418
1. Because the compiler needs to allow for the fact that the type might use a constant-specific class body (see API doc)
2. How would you use that?
3. That's the way Enum is defined
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Expert Comment

by:objects
ID: 33516429
rotfl :D
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Author Comment

by:Jonah11
ID: 33516471
objects, thanks for that explanation of 1.

CEHJ, regarding point 2, I was thinking, in analogy with the example in that link, that something like:

 K newKey = keyType.getClass().valueOf(oldKey);

might work, but it does not.

Regarding 3, I understand that is the definition, but the definition seems paradoxical to me.  That is, looking at the definition of Enum itself we have:

Class Enum<E extends Enum<E>>

which is a generic definition, but the objects allowed in the generics must extend Enum<E>, but what is the definition of Enum<E>?  Won't this give us an infinite regress?

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Expert Comment

by:objects
ID: 33516504
>  K newKey = keyType.getClass().valueOf(oldKey);

thats calling valueOf() on the Class instance, not the enum.
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Author Comment

by:Jonah11
ID: 33516608
objects, okay, bear with me.  The line from the example I linked is:

ap = Apple.valueOf("C");

where Apple is the enum class in question.  So I am still not seeing the difference.  Thanks.
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Expert Comment

by:objects
ID: 33516661
>  K newKey = keyType.getClass().valueOf(oldKey);

getClass() returns a Class object (not an Enum object)
Class does not have a valueOf() method
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Author Comment

by:Jonah11
ID: 33516761
ah ok, thanks.

finally, did you have an explanation of point 3, my question about the seeming infinite regress?
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Author Comment

by:Jonah11
ID: 33516801
Also, I cannot seem to find any documentation on the valueOf(String) method used in that example -- all I see is this:

http://download.oracle.com/javase/1.5.0/docs/api/java/lang/Enum.html#valueOf%28java.lang.Class,%20java.lang.String%29

which is a different method.
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Assisted Solution

by:objects
objects earned 250 total points
ID: 33516802
Its saying extenders of Enum can only pass themselves as E

some more reading on enums

http://madbean.com/2004/mb2004-3/
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Expert Comment

by:objects
ID: 33516807
> Also, I cannot seem to find any documentation on the valueOf(String) method used in that example -- all I see is this:

thats a special case as I mentioned earlier
the compiler creates it, the link above discusses it I think
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Author Comment

by:Jonah11
ID: 33517105
objects,

that is a great link, it answered all my questions.  thank you.  
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Expert Comment

by:objects
ID: 33517134
Amazed how often people post code on this site that they are unable to actually explain
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Expert Comment

by:CEHJ
ID: 33518899
:-)
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