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Groups > comp.lang.java.programmer > #38891

Re: Store Suppliers in a map

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Responding to mike:
> I need to to different suppliers:
> 
> - Supplier<String>
> - Supplier<Integer>
> - Supplier<List<String>>
> 
> Map<String, Supplier> map= new HashMap<>();
> 
> How can I define a map ( some similar collection ) to store these different suppliers in? 
> 
> When I get it from map what will be the type?

You need a uniform key type. That leaves only raw Supplier or wildcard
Supplier<?> or similar, just as you put it above. Thus the compile-time
key type you receive from the map will be raw Supplier or wildcard
Supplier<?>, as well.

Just assuming that through some magic you'd be able to actually yank a
compile-time Supplier<String> from the map - how would you proceed
working with it? The client code that queries the map would probably be
generic in the Supplier's result type, anyway, so you wouldn't have
gained much.

You could try to use an HList[1] or some abstraction on top of it. I
haven't used those in Java, yet. From my limited experience in Scala I'd
say this isn't for the faint of heart, and I wouldn't expect it to be
more friendly in Java - see [2]. :)

Or you could bury the whole remaining computation, starting from the
Supplier, in a custom data type and use this as a key type. "Inside"
this type, the proper type would be known then, generically or statically.

There may be other, less cumbersome approaches, depending on what you
actually want to achieve.

Best regards,
Patrick

[1]
http://www.functionaljava.org/javadoc/4.8.1/functionaljava/fj/data/hlist/HList.html
[2] http://www.functionaljava.org/examples-java7.html#hlistAppend

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Thread

Store Suppliers in a map mike <mikaelpetterson@hotmail.com> - 2019-04-09 22:01 -0700
  Re: Store Suppliers in a map mike <mikaelpetterson@hotmail.com> - 2019-04-09 22:04 -0700
  Re: Store Suppliers in a map Patrick Roemer <sangamon@netcologne.de> - 2019-04-10 16:12 +0200
    Re: Store Suppliers in a map Patrick Roemer <sangamon@netcologne.de> - 2019-04-10 16:35 +0200
  Re: Store Suppliers in a map Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2019-04-10 19:07 +0200

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