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

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