Theo quan điểm cá nhân Abtract là để dùng khi thiết kế Object (Status, Behaviour)
Còn Interface là để dùng khi khai báo hành vi (Behaviour).
Một Object thì có những hành vi. Có thể nói, Interface là chia nhỏ hơn của Object. Nó có mặt cũng để phục vụ cho Object.
Về mặt tư tưởng là như vậy, còn về mặt cú pháp thì trích một câu trả lời ở SOF.
The key technical differences between an abstract class and an interface are: * Abstract classes can have constants, members, method stubs (methods without a body) and defined methods, whereas interfaces can only have constants and methods stubs. * Methods and members of an abstract class can be defined with any visibility, whereas all methods of an interface must be defined as public (they are defined public by default). * When inheriting an abstract class, a concrete child class must define the abstract methods, whereas an an abstract class can extend another abstract class and abstract methods from the parent class don't have to be defined. * Similarly, an interface extending another interface is not responsible for implementing methodsfrom the parent interface. This is because interfaces cannot define any implementation. * A child class can only extend a single class (abstract or concrete), whereas an interface can extend or a class can implement multiple other interfaces. * A child class can define abstract methods with the same or less restrictive visibility, whereas a class implementing an interface must define the methods with the exact same visibility (public).