Abstract class vs Interface

Abstract class

• Abstract class can't be instantised. It can only be inherited.

  Because there are abstract functions in the abstract class.

• Abstract class has attributes and methods.

• The class that inherits the abstract class must override the abstract functions.

When a class inherits an abstract class, there is the is-a relationship between the classes

The advantages are:

• reuse the code. (DRY)

• avoid initializing the parent class accendentally.

• avoid forgetting to implement the methods.

Interface

• Interface has methods, but no attributes

• Interface's methods can't be implemented.

• The class that implements the interface must implement all the methods in the interface.

When a class implements an interface, there is the has-a relationship between the classe and the interface.

The advantages are:

• decoupling the codes:

  The interface declares the methods of the class. The use only needs to know what methods can be used, doesn't need to think about how these methods are implemented.

Practice

• When we want to reuse code, we think about the abstract class. When we want to abstract the code, we look for the interface.

• Usually, we first create child class, then create abstract parent class, while we first design the interface then write the class.

Programming based on interface rather than implementation

1. The naming of the function cannot reveal any implementation details.

   use update() rather than AwsUpdate()

2. Encapsulate specific implementation details.

   set createAwsBucket() to private.

3. We define the interfaces for classes, so that the user of classes depends on the interfaces rather than the classes themselves while coding.