Path: csiph.com!news.dns-netz.com!news.freedyn.net!newsreader4.netcologne.de!news.netcologne.de!.POSTED.2001-4dd3-ae38-0-ada1-1290-a847-1871.ipv6dyn.netcologne.de!not-for-mail
From: Patrick Roemer <sangamon@netcologne.de>
Newsgroups: comp.lang.java.programmer
Subject: Re: Store Suppliers in a map
Date: Wed, 10 Apr 2019 16:12:50 +0200
Organization: news.netcologne.de
Distribution: world
Message-ID: <q8ktl2$g7$1@newsreader4.netcologne.de>
References: <88a3b286-0d96-425e-ac82-c02e3ebcc84a@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Injection-Date: Wed, 10 Apr 2019 14:12:50 -0000 (UTC)
Injection-Info: newsreader4.netcologne.de; posting-host="2001-4dd3-ae38-0-ada1-1290-a847-1871.ipv6dyn.netcologne.de:2001:4dd3:ae38:0:ada1:1290:a847:1871"; logging-data="519"; mail-complaints-to="abuse@netcologne.de"
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.24) Gecko/20100411 Thunderbird/2.0.0.24 Mnenhy/0.7.6.0
In-Reply-To: <88a3b286-0d96-425e-ac82-c02e3ebcc84a@googlegroups.com>
Content-Language: en-US
Xref: csiph.com comp.lang.java.programmer:38891

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