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From: =?UTF-8?Q?Arne_Vajh=c3=b8j?= <arne@vajhoej.dk>
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Subject: Re: Why does Java argue about making a class instance
Date: Mon, 28 Mar 2016 10:10:12 -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

Other have already pointed out the problem.

But let me try and elaborate.

Or maybe add to the confusion.

:-)

When a class is declared inside another class with static then
it is just another class with an unusual name and more complex
accessibility.

Example:

public class Main1 {
	// note static
	public static class Foobar {
		@Override
		public String toString() {
			return super.toString();
		}
	}
	public void test() {
		// full syntax
		Main1.Foobar a = new Main1.Foobar();
		System.out.println(a);
		// abbreviated syntax
		Foobar b = new Foobar();
		System.out.println(b);
	}
	public static void main(String[] args) {
		Main1 o = new Main1();
		o.test();
	}
}

When it is declared without static, then each instance
belong to an instance of the outer class. And in fact code
can refer to that using classname.this syntax.

Example:

public class Main2 {
	// note not static
	public class Foobar {
		@Override
		public String toString() {
			return super.toString() + "/" + Main2.this.toString();
		}
	}
	public void test() {
		// full syntax
		Main2.Foobar a = this.new Foobar();
		System.out.println(a);
		// abbreviated syntax
		Foobar b = new Foobar();
		System.out.println(b);
	}
	public static void main(String[] args) {
		Main2 o = new Main2();
		o.test();
	}
}

(and your problem as pointed out by several other was that you had
this case and were using the abbreviated syntax with an implicit this
prefix on new - an getting the error because in the static main there
is no this)

Arne