Creating an EnumMap in a generic

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Mick - 23 Jun 2006 11:11 GMT

Hello, I'm wondering if anyone has ever tried to create an EnumMap
object within a generic, such that the EnumMap keyed based off the
class parameter for that generic. Here is a sample of code that I
thought would logically work for this problem:

START_SNIPPET

public class GenericClass {

private Map<P,String> properties;

public GenericClass() {
properties = new EnumMap<P,String>( new HashMap<P,String>() );
// compile error
}

END_SNIPPET

Eclipse points out the following compile error directly on the first
'P' in the marked line of code: "Bound mismatch: The type P is not a
valid substitute for the bounded parameter <K extends Enum<K>> of the
type EnumMap<K,V>"

Does anyone know what I am doing wrong, or perhaps a better way to
accomplish what I am looking for? I realize I could just use a HashMap
for this situation, but I'm curious to know if what I'm attempting is
possible. It seems like it should be possible to use an EnumMap in
this situation.

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Thomas Fritsch - 23 Jun 2006 12:29 GMT

> START_SNIPPET

import java.util.*;

> public class GenericClass {

public class GenericClass> {

> private Map<P,String> properties;
>
> public GenericClass() {
> properties = new EnumMap<P,String>( new HashMap<P,String>() );
> // compile error
> }

}

> END_SNIPPET
>
> Eclipse points out the following compile error directly on the first
> 'P' in the marked line of code: "Bound mismatch: The type P is not a
> valid substitute for the bounded parameter <K extends Enum<K>> of the
> type EnumMap<K,V>"

In your first line you have declared P to be

Hence the compiler complains. To make it happy you have to declare
>

> Does anyone know what I am doing wrong, ...

Signature

Thomas

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Mick - 23 Jun 2006 13:15 GMT

> In your first line you have declared P to be
> 
> Hence the compiler complains. To make it happy you have to declare
> >

That's exactly what I was looking for; thank you.

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Mick - 23 Jun 2006 22:58 GMT

Apparently this solution does not work. You cannot create a new
EnumMap based off an empty HashMap regardless of the types used.

Reply to this Message

Roland de Ruiter - 24 Jun 2006 12:16 GMT

> Apparently this solution does not work. You cannot create a new
> EnumMap based off an empty HashMap regardless of the types used.

You could pass the class denoting the key type as parameter to your
constructor (like EnumMap<K extends Enum<K>, V> has a contructor with
Class<K> as parameter):

public class GenericClass>
{
private Map<P,String> properties;
public GenericClass(Class keyType)
{
properties = new EnumMap<P,String>( keyType );
}
}

Usage example:

GenericClass<Thread.State> foo =
new GenericClass<Thread.State>(Thread.State.class);

Signature

Regards,

Roland

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Mick - 25 Jun 2006 02:46 GMT

> You could pass the class denoting the key type as parameter to your
> constructor (like EnumMap<K extends Enum<K>, V> has a contructor with
> Class<K> as parameter):

> --
> Regards,
>
> Roland

That makes a lot of sense, and it made me realize why EnumMap has that
seemingly silly constructor in the first place. I had moved on happily
with a plain-old HashMap, but I'll make a note to go revisit that code.

Thank you!

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Creating an EnumMap in a generic

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