Classes (OOP) (OCR A Level Computer Science) : Revision Note

Craig Godbold

Written by: Craig Godbold

Reviewed by: James Woodhouse

Updated on

Classes (OOP)

What is a class?

  • Classes are used as blueprints or templates that can be used to create objects within Object Oriented Programming or OOP

  • An object is created from a specific instance of a class and has its own state and behaviours

  • Using this method allows for reusable and organised code in a modular way

  • Consider a class of students

    • Each student has a name, a date of birth and gender

  • Therefore we can create a blueprint or template for all students by making a class which contains these three attributes

  • As each student (or object) is created they will each have their own name, date of birth and gender attributes along with their own state for example

    • Name “John”

    • Date of birth “06/10/2015”

    • Gender “Male”

  • Some classes are already prebuilt into a programming language saving the developer from having to write them from scratch and often provide common functionality

  • Examples from Java include:

    • Date and calendar when working with dates

    • String when working with strings of text

    • Random when generating random numbers

    • Scanner when reading input from a user or file

  • Custom classes are created by the programmer to define new data types

  • For example, a class for animals does not exist and so the programmer must define a custom class

  • Instantiation is the term used for creating an object from a class

  • Each class contains attributes which are essentially variables within a class and are also known as Class Variables

  • Objects that are created from a class contain attributes which are also known as instance variables

  • Classes can also contain methods/functions/procedures

  • Methods are actions or behaviours that can be performed

  • The name that is used to refer to an object is known as the identifier

  • Below is a visual representation of both a class and objects that have been instantiated

  • In the image below, the identifiers are P1 and P2

Example of a class and objects

Example of a class and objects

Examiner Tips and Tricks

  • Although you may see the terms methods/functions/ procedures, a method can be either a function or a procedure

  • A function is a method that must return a value

  • A procedure is a method that does not need to return a value

Worked Example

What is a class in object-oriented programming (OOP) ?   

[2]

How to answer this question:

  • Classes provide a way to organize code in a modular way [1]

  • A blueprint or template for creating objects with specific attributes [1]

Answer:

 Example answer to get full marks:

A class is acts as a blueprint or template for creating objects with specific attributes [1 mark], while also providing a way to organize code in a modular fashion. [1 mark]

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Craig Godbold

Author: Craig Godbold

Expertise: Computer Science Content Creator

Craig is an experienced educator with 14 years of teaching experience in the UK, USA, and China, He has held a variety of roles in education, including Head of Faculty, IB teacher and now currently teaches A level and GCSE courses. Craig supported his students to achieve the best results the school had ever had in IB Computer Science.

James Woodhouse

Reviewer: James Woodhouse

Expertise: Computer Science Lead

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.

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