Lecture 13

Fall 2018

Narges Norouzi

Recap

• Static
  • Static field and static method
• Final
  • Final class, method, and variable

Abstract behavior

football game

Abstract behaviors

abstract behaviors (1)

• Does all football objects kick the ball?
  • No. So “to kick” is not a behavior of football objects.
• All football objects needs to be drawn
  • “to draw” is a behavior of all objects.
  • But what is the shape of a football object?
  • It depends to the specific type of instance.

abstract behaviors (2)

• “draw” is an abstract behavior of a football object
  • But we cannot specify how to draw football objects
  • It depends on the specific class of football objects
• “draw” is a concrete behavior of Ball
• “kick” is not a behavior of a football object but it is a concrete behavior of a Player object

Remember shapes?

Abstract shape

• Shape is an abstract class
  • Some methods aren't implemented in Shape
  • Those methods should be implemented in subclasses
  • Example: getArea() - getPerimeter()
• These methods are abstract methods

abstract methods (1)

• Shape class
  • getArea() and getPerimeter()
• FootballObject
  • draw()
• getCoordinates() is not an abstract method
• How should we implement an abstract method?

abstract methods(2)

• Abstract method: no implementation
• A class containing abstract methods:
  • an abstract class
• You cannot instantiate any object from an abstract classes. Why?
• If a subclass doesn't implement all abstract methods of its superclass, it will be abstract too

example

Interface

Interface

• Sometimes we have an abstract class, with no concrete method

• Interface:

  • Pure abstract class

  • No concrete method

Interface

Methods in interfaces

• All the methods are implicitly abstract
  • ​No need to specify them as abstract
• All the methods are implicitly public
  • ​No need to write public access specifier

Interface implementatioN

• A class may inherit from an abstract class

• A class may implement interfaces​

• Is-a relation exists if a class implements an interface

• What if it does not implement all the methods?

  • The class becomes an abstract class

Zybooks chapters we covered

• Chapter 11: abstract class and interfaces