Throw exception when second instance is created

Question

I recently came across this question and have not been able to find a solution to it. I hope I will get a convincing answer here.

Question : Let A be a non abstract class which is extended (separately) by two classes B and C. Now the task is to allow only one instance of the these classes to be created. If the client code tries to create a second instance of any of these classes, then an exception should be thrown.

A obj11 = new A(); // Fine.

A obj12 = new A(); // thrown an exception. 

B obj21 = new B(); // OR 

A obj21 = new B(); // both these should be fine

But shouldn't allow 2nd instance of class B to be created.

Thanks in advance.

P.S. : This is different from singleton.

Answer 1

The following should do it. In the real world there should be some synchronized keywords. All crammed into one file for simplicity.

public class A {

   static HashSet<Class> thereCanBeOnlyOne = new HashSet();

   public A() {
      Class c = this.getClass();
      if (thereCanBeOnlyOne.contains(c))
         throw new RuntimeException();
      thereCanBeOnlyOne.add(c);
   }



   static public class B extends A {}

   static public class C extends A {}


   public static void main(String[] args) {
      A a1 = new A();
      B b1 = new B();  // OK
      C c1 = new C();  // OK

      C c2 = new C();   // throws exception

   }
}