About OOPS

When you are designing an entity which can have more than one implementation, the goal is to concentrate on behaviour.So When you have a common implementation, use abstract classes, with which you can enforce an implementation hierarchy and avoid duplicate code. Using abstract classes, however, should not affect your decision to use interfaces to define your behaviour. One should rely on interface to define the behaviour of an entity, it gives more generalization space. For long terms where behaviour might keep changing, using abstract class should be avoided or at least the abstract class should implement an interface so that behavioural changes can be incorporated with minimum changes.

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