// 一個簡單的抽象類
abstract class A {
  abstract void callme();
  // concrete methods are still allowed in abstract classes
  void callmetoo() {
    System.out.println("This is a concrete method.");
  }
}
class B extends A {
  void callme() {
    System.out.println("B's implementation of callme.");
  }
}
class AbstractDemo {
  public static void main(String args[]) {
    B b = new B();
    b.callme();
    b.callmetoo();
  }
}