Path: csiph.com!optima2.xanadu-bbs.net!xanadu-bbs.net!nntp.club.cc.cmu.edu!micro-heart-of-gold.mit.edu!bloom-beacon.mit.edu!bloom-beacon.mit.edu!panix!not-for-mail
From: Popping mad <rainbow@colition.gov>
Newsgroups: comp.lang.java.programmer
Subject: Why does Java argue about making a class instance
Date: Sat, 26 Mar 2016 19:44:53 +0000 (UTC)
Organization: PANIX Public Access Internet and UNIX, NYC
Lines: 57
Message-ID: <nd6orl$k7m$1@reader1.panix.com>
NNTP-Posting-Host: www.mrbrklyn.com
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Trace: reader1.panix.com 1459021493 20726 96.57.23.82 (26 Mar 2016 19:44:53 GMT)
X-Complaints-To: abuse@panix.com
NNTP-Posting-Date: Sat, 26 Mar 2016 19:44:53 +0000 (UTC)
User-Agent: Pan/0.139 (Sexual Chocolate; GIT bf56508 git://git.gnome.org/pan2)
Xref: csiph.com comp.lang.java.programmer:34024

When I try to compile this (example from antlr4 FIWIW)

import org.antlr.v4.runtime.ANTLRInputStream;
import org.antlr.v4.runtime.CommonTokenStream;
import org.antlr.v4.runtime.tree.ParseTree;
import org.antlr.v4.runtime.tree.TerminalNode;
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
		 ANTLRInputStream input = new ANTLRInputStream("y = 6*4 + 
9 /8 / 2.5 + -3.21 * RUBEN; C = 5/9 * F + 32;");
		 CalcLexer lexer = new CalcLexer(input);
		 CommonTokenStream tokens = new CommonTokenStream(lexer);
		 CalcParser parser = new CalcParser(tokens);
		 CollectVarsVisitor my_visitor = new CollectVarsVisitor();
//		 parser.prog();
		 ParseTree tree  = parser.prog();
		 tree.accept(my_visitor);
	 	 System.out.println(my_visitor.vars);
    }

	 public class CollectVarsVisitor extends CalcBaseVisitor<Void> {
	 	HashSet<String> vars = new HashSet<String>();
		@Override
    	public Void visitStmt(CalcParser.StmtContext ctx) {
		TerminalNode id = ctx.ID();
       if(id != null) {
           vars.add(id.getText());
       }
       return super.visitStmt(ctx);
    	}

	 	@Override
    	public Void visitExpr(CalcParser.ExprContext ctx) {
        TerminalNode id = ctx.ID();
        if(id != null) {
            vars.add(id.getText());
        }
        return super.visitExpr(ctx);
    	}
	 }
}

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