Both abstract classes and Interfaces have one goal in common. They should be used when the implementation is not specific. To make things clear let us say you have a 'Shape' class. Shapes could be of different kinds for e.g. a circle, a rectangle, a square etc. If 'Shape' class had a method called 'draw()', during designing you class, you cannot specifically say how this method should be defined. That's because each shape is drawn differently. So you have a draw method for square, a rectangle etc. Here both abstract classes and interfaces can be used. Now if you want to enforce super-class sub-class relationship between shape and square and other classes you use abstract classes. Whereas in interfaces such relationship is not enforced. Hope it's clear now..
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