22.3. Interfaces and Abstract Classes

We mentioned previously that interfaces share some characteristics with abstract classes. Recall that an abstract class is one declared with the abstract keyword. You may not create an object from an abstract class and, like an interface, an abstract class is allowed to contain methods that only have signatures (that is, methods that don’t have implementation code).

The main differences between interfaces and abstract classes are:

1. You implement an interface, while you extend an abstract class. The net effect of this is that a class may implement interfaces while also extending a class. Note that while you can implement more than one interface, you can only extend one class.

2. Abstract classes may contain non-constant fields, while interfaces can only contain constant fields.

3. Abstract classes should be used to collect and specify behavior by related classes, while an interface should be used to specify related behaviors that may be common across unrelated classes.

   Example

   Think about our IFeedable interface. If we want to add a Dog class to our application, we might implement the IFeedable interface for our Dog class. This makes sense as dogs are creatures that we feed. However, as dogs and cats are so different, it is unlikely that they would share many characteristics through a Pet class.

22.3.1. Check Your Understanding

Question

Check all statements that are TRUE about the differences between interfaces and abstract classes.

1. You extend an abstract class, but implement an interface.

2. You can implement many interfaces and many classes.

3. Interfaces cannot contain non-constant fields, but abstract classes can.

4. Methods that use instance properties can be in both interfaces and abstract classes.