java - Why do I get "non-static variable this cannot be referenced from a static context"?

Question

I have a very simple class which I want to use as a subclass of another one. But when I put its code in the parent's class I get :

non-static variable this cannot be referenced from a static context

On the other hand when I put the sublass GenTest's class code outside the the "parent's" class code - JavaApp1 I do not get this error.

public class JavaApp1 {

    class GenTest {  
        @Deprecated
        void oldFunction() {
            System.out.println("don't use that");
        }
        void newFunction() {
            System.out.println("That's ok.");
        }
    }

    public static void main(String[] args) {
        GenTest x = new GenTest();
        x.oldFunction();
        x.newFunction();
    }
}

Why is this happening ?

Answer 1

Your nested class (which isn't a subclass, by the way) isn't marked as being static, therefore it's an inner class which requires an instance of the encoding class (JavaApp1) in order to construct it.

Options:

• Make the nested class static
• Make it not an inner class (i.e. not within JavaApp1 at all)

• Create an instance of JavaApp1 as the "enclosing instance":

  GenTest x = new JavaApp1().new GenTest();
  

Personally I'd go with the second approach - nested classes in Java have a few oddities around them, so I'd use top-level classes unless you have a good reason to make it nested. (The final option is particularly messy, IMO.)

See section 8.1.3 of the JLS for more information about inner classes.