Java Stack overflow

1)  Access to superclass private fields using the super keyword in a subclass

I am completely fine with this working. I also noticed that it seems to work with data being marked as protected instead of private and not using the super keyword. I am mostly interested in any documentation mentioning this ability of the super keyword.

abstract class OuterClass {


    protected class NestedSuperClass<T> {
        private T data;

        public NestedSuperClass (T t) {
            this.data = t;
        }

        public T getOutput() {
            return data;
        }
    }

    protected class NestedSubClass<T> extends NestedSuperClass<T> {
        public NestedSubClass (T t) {
            super(t);
        }

        protected void setOutput(T t) {
            super.data = t;
        }
    }
}