First answer:
Use polymorphism for method overridding and method overloading. Other class methods used in different class then two options: first method inherited, second method over written. Here extend interface: use them, or implemention method: logic write them. Polymorphism used for method, class inheritance.
Second answer:
Is there any difference between cm.start(); and rf.start();?
Yes, both are objects that are completely different with respect to each other. Do not create interface objects because Java doesn`t support interface objects. First object created for interface and second for Refrigerator class. Second object right now.
