From Java How to Program about abstract classes:
Because they’re used only as superclasses in inheritance hierarchies,
we refer to them as abstract superclasses. These classes cannot be
used to instantiate objects, because abstract classes are incomplete.
Subclasses must declare the “missing pieces” to become “concrete” classes,
from which you can instantiate objects. Otherwise, these subclasses, too,
will be abstract.
To answer your question “What is the reason to use interfaces?”:
An abstract class’s purpose is to provide an appropriate superclass
from which other classes can inherit and thus share a common design.
As opposed to an interface:
An interface describes a set of methods that can be called on an
object, but does not provide concrete implementations for all the
methods… Once a class implements an interface, all objects of that class have
an is-a relationship with the interface type, and all objects of the
class are guaranteed to provide the functionality described by the
interface. This is true of all subclasses of that class as well.
So, to answer your question “I was wondering when I should use interfaces”, I think you should use interfaces when you want a full implementation and use abstract classes when you want partial pieces for your design (for reusability)