Inner Class extending the outer class

Question

Could anyone explain this code to me? It can compile and run. Could anyone tell me what the class Main.Inner.Inner is? What members does it consists of? How the compiler manage to build such a class?
public class Main {
    public static class Inner extends Main {
    public static void main(String[] args) {
        System.out.println("Hello, world.");
}

Answer 1

By the way, it's not really an inner class, because it's static. It's just a nested class.
You might want to have a nested class extend its enclosing class if, maybe, you wanted to delegate to subclasses and didn't want to create a lot of extra source code files, which might make sense if the nested subclasses were really small.
public abstract class Animal {
  public abstract void makeSound();
  private Animal() {} // can't be directly instantiated
  private static class Dog extends Animal {
    public void makeSound() { System.out.println("woof"); }
  private static class Cat extends Animal {
    public void makeSound() { System.out.println("meow"); }
  private static class Zebra extends Animal {
    public void makeSound() { System.out.println("i am a zebra"); }
  public static Animal get(String desc) {
    if ("fetches sticks".equals(desc)) {
      return new Dog();
    if ("hunts mice".equals(desc)) {
      return new Cat();
    if ("stripey horse".equals(desc)) {
      return new Zebra();
    return null;
public class AnimalTest {
  public static void main(String... argv) {
    Animal a = Animal.get("fetches sticks");
    a.makeSound();
}