Classes (OOP) (Cambridge International A-Level Computer Science)

Classes (Object-Oriented Programming)

Spec point — 20.1.5

What is a class?

• A class is a blueprint used to create objects in Object-Oriented Programming (OOP)
• It defines a structure for attributes (data) and methods(behaviour)

Key terms

Example

• A Student class could contain:

  • Attributes: name, dateOfBirth, gender
  • Each Student object (e.g. John, 06/10/2015, Male) has its own values

Prebuilt vs custom classes

• Below is a visual representation of both a class and objects that have been instantiated
• In the image, the identifiers are P1 and P2

Example of a class and objects

Programming classes (OOP)

How do you program a class?

• In this example we will:

  • Define a class for a car sales program where the following is required:

    • Name of Manufacturer
    • Model of Car
    • Price
    • Mileage
    • Whether the car is preowned

• Instantiate two objects for the car sales program ensuring that they have appropriate identifiers

• List two methods that would be useful for a car object

Pseudocode

Pseudocode for creating classes, a constructor, and functions

Java

Python

Worked Example A computer game is being designed that will include different vehicles. A prototype for the game is being developed using object‑oriented programming.

The class Vehicle stores data about the vehicles. Each vehicle has an identification name, a maximum speed, a current speed and a horizontal position. The value IncreaseAmount is added to the current speed each time the vehicle increases its speed.

Write program code to declare the class Vehicle. All attributes must be private.

You only need to declare the class and its constructor. Do not declare any other methods.

Use your programming language’s appropriate constructor.

If you are writing program code in Python, include attribute declarations using comments. [5]

Final answer: Answer

• Class header (and close where appropriate) [1 mark]
• 5 (private) attribute declarations including data types [1 mark]
• Constructor header (and close where appropriate) taking 3 parameters (min) [1 mark]
• Assigning ID, MaxSpeed and IncreaseAmount to parameters [1 mark]
• Assigning CurrentSpeed and HorizontalPosition to 0 [1 mark]

Example program code:

VB.NET

Java

Python