Interfaces

Interfaces allow us to avoid hardcoding features in an application. We can move specific implementation details into subclasses, and then use an interface to communicate between the application and the subclasses.

Interfaces vs Abstract Classes

Here are some of the similarities and differences between abstract classes and interfaces:

Abstract class

• Can have class variables.
• Can have both abstract methods and concrete methods that are shared with the subclasses.
• Can have instance variables, i.e. variables that are specific to individual subclasses.
• Subclasses can only extend one class.

Interfaces

• Can have class variables.
• Every method in an interface is abstract.
• Cannot have instance variables. Variables in an interface must be the same for every class implementing the interface.
• Classes can implement more than one interface and have multiple inheritance.

When to Use an Interface

We use an interface when:

• We expect unrelated classes will be implementing our interface.
• We want to support multiple inheritance.
• We want to specify the behavior for a data type, but we do not care about the implementation.

QUIZ QUESTION::

For each of the descriptions below, does it better describe and interface or an abstract class?

ANSWER CHOICES:

Abstract class

Abstract class

Abstract class

Interface

Interface

Description	Interface or Abstract class
We expect unrelated classes will be implementing our super class.
We want to support multiple inheritance.
We want subclasses to have instance variables.
We want to provide concrete methods for subclasses
We want to make use of both concrete methods and abstract methods for subclasses.

SOLUTION:

Description	Interface or Abstract class
We want subclasses to have instance variables.	Abstract class
We want to provide concrete methods for subclasses	Abstract class
We want to make use of both concrete methods and abstract methods for subclasses.	Abstract class
We want subclasses to have instance variables.	Abstract class
We want to provide concrete methods for subclasses	Abstract class
We want to make use of both concrete methods and abstract methods for subclasses.	Abstract class
We want subclasses to have instance variables.	Abstract class
We want to provide concrete methods for subclasses	Abstract class
We want to make use of both concrete methods and abstract methods for subclasses.	Abstract class
We expect unrelated classes will be implementing our super class.	Interface
We want to support multiple inheritance.	Interface
We expect unrelated classes will be implementing our super class.	Interface
We want to support multiple inheritance.	Interface