Lets understand it with the following simple example. This happens because this is NON-STATIC INNER CLASS. You should need the instance of outer class.
public class PQ {
public static void main(String[] args) {
// create dog object here
Dog dog = new PQ().new Dog();
//OR
PQ pq = new PQ();
Dog dog1 = pq.new Dog();
}
abstract class Animal {
abstract void checkup();
}
class Dog extends Animal {
@Override
void checkup() {
System.out.println("Dog checkup");
}
}
class Cat extends Animal {
@Override
void checkup() {
System.out.println("Cat Checkup");
}
}
}