Interfaces can never be directly instantiated.
extracur student1=new extracur(); // not possible
And you should capitalize interface names. You need instead:
Extracur student1 = new Extracur() {
// implement your methods
};
Explanation: The code does not instantiate the interface, but an anonymous inner class which implements the interface.
You should also understand that the compiler throws an error at compile time while you are trying to catch an error at runtime (too late in this case).