You can offer the best of both worlds, an interface and an 
abstract class. Implementors can ignore your abstract class 
if they choose. The only drawback of doing that is calling 
methods via their interface name is slightly slower than 
calling them via their abstract class name.

Interface is use where we known that all these functions 
which are declared in interface is implemented somewhere if 
we call that interface.and we can use abstract class where 
we don't known how it is implemented in call function.