inner class able to access outer classes attributes from method only

Question

Please review this code which compiles fine :

class OuterClass {

    String a = "A";
    String b = "B";
    String c = "C";


    public static class StaticInnerClass {

    }

    public String stringConCat() {
        return a + b + c;
    }
}

this is as i expect as the inner class can access the outer classes attributes. But now when i try the same code but assigning a outer attribute to inner attribute the system complains:

class OuterClass {

    String a = "A";
    String b = "B";
    String c = "C";


    public static class StaticInnerClass {
        String x = a; //this can not be done, why ?
    }

    public String stringConCat() {
        return a + b + c;
    }
}

the error/warning at compile time is: Non-static field a cannot be referenced from a static context.

is it because in the method stringConCat() you actually need an instance to call the method (post constructor call) so its allowed ? Whereas, in the second example there is no real instance so it treats it as a static reference ?

I've read I thought inner classes could access the outer class variables/methods? but its still not sinking. Can someone help?

Answer 1

The key is in your error message: "Non-static field a cannot be referenced from a static context."

inner classes can access outer class variables, but your nested class is static, not inner, and the variable is not static. Either make the variable static, or make the nested class non-static.