What causes error "No enclosing instance of type Foo is accessible" and how do I fix it?

Question

I have the following code:

class Hello {     class Thing {         public int size;          Thing() {             size = 0;         }     }      public static void main(String[] args) {         Thing thing1 = new Thing();         System.out.println("Hello, World!");     } } 

I know Thing does nothing, but my Hello, World program compiles just fine without it. It's only my defined classes that are failing on me.

And it refuses to compile. I get No enclosing instance of type Hello is accessible." at the line that creates a new Thing. I'm guessing either:

1. I have system level problems (either in DrJava or my Java install) or
2. I have some basic misunderstanding of how to construct a working program in java.

Any ideas?

Answer 1

static class Thing will make your program work.

As it is, you've got Thing as an inner class, which (by definition) is associated with a particular instance of Hello (even if it never uses or refers to it), which means it's an error to say new Thing(); without having a particular Hello instance in scope.

If you declare it as a static class instead, then it's a "nested" class, which doesn't need a particular Hello instance.