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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Why does Java argue about making a class instance
Date: Sat, 26 Mar 2016 16:04:07 -0400
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On 3/26/2016 3:44 PM, Popping mad wrote:
> When I try to compile this (example from antlr4 FIWIW)
> [...]
> public class Main {
>      public static void main(String[] args) {
>[...]
> 		 CollectVarsVisitor my_visitor = new CollectVarsVisitor();
> [...]
>      }
>
> 	 public class CollectVarsVisitor extends CalcBaseVisitor<Void> {
> [...]
>      	}
> [...]

> it says
>
> [ruben@manjaro calculator]$ javac Main.java
> Main.java:15: error: non-static variable this cannot be referenced from a
> static context
>                   CollectVarsVisitor my_visitor = new CollectVarsVisitor();
>                                                   ^
> 1 error
>
> I have no clue what is the problem, but it is not a standard static
> problem

     CollectVarsVisitor is an "inner class," a nested class each of whose
instances belongs to an instance of the containing class (Main).  You
cannot have a CollectVarsVisitor instance without a Main instance to
own it.  Since the main() method is static, there is no Main instance
around to be the owner of the new CollectVarsVisitor.  And that's why
the compiler complains (in its ever-so-transparent way).

     I'm not quite sure what you're trying to do, but it looks like the
solution might be to use a static nested class:

	public /* => */ static /* <= */ class CollectVarsVisitor ...

If the nested class is truly supposed to be an inner class (belonging
to a Main instance), then you'll need a Main:

	Main my_main = new Main();
	CollectVarsVisitor my_visitor
	    = my_main.new CollectVarsVisitor();

-- 
esosman@comcast-dot-net.invalid
"Don't be afraid of work. Make work afraid of you." -- TLM