Servlet and Interface

Question

Hi,
Why would I have to instantiate a object for interface to access and execute it's method from Servlet?
For ex: Servlet A refers to Interface B which has method bb. The implementation class is C.
Inside Serverlet A, if I refer to (refer to below line..)
B b;
int z = b.bb(); //as the method bb returns integer value.
The Tomcat application server throws, NullPointerException for accessing b.bb. I realized that, bb method was not accessable for servlet A.
Now I tried to initialize the implementation class C (both B b= new C() and also I tried C c= new C()) and then access the bb method i.e. int z = b.bb() OR int z = c.bb().
Then the servlet runs without any problem. I was expecting that, there is no need to expose and initialize implementation class to any other application/class.
Does anyone have any idea?
TIA,
Tom

Answer 1

In the first case the reference 'b' is not pointing to any object. So invoking method on it throws null pointer exception. In the second case, since you have b pointing to an object(here of type C which implements B)
you have an implementation (non-null ref) and so you can invoke methods without NPE.
The bottom line is: Invoking instance methods on null references causes NullPointerException. And since you cannot instantiate an interface directly - you have to invoke the method on an object of a class that implements the interface.