Classes and objects, attributes and methods, instantiation, and how the object oriented paradigm models real-world entities.

What this dot point is asking

CCEA wants you to define the building blocks of the object oriented paradigm: a class, an object, an attribute and a method. You must explain instantiation, distinguish a class (the template) from an object (the instance), and show how grouping data and behaviour together models a real-world entity. These ideas underpin every program you write in this course, so they are tested directly and assumed in later questions.

The answer

Classes and objects

A useful way to picture it: the class is the architect's plan for a house, and each object is an actual house built from that plan. Every house has the same rooms defined by the plan, but each has its own paint colour, furniture and occupants, just as each object has its own attribute values.

Attributes and methods

An attribute is a named data item that belongs to an object and stores part of its data. The collection of an object's attribute values at a given moment is called its state. A method is a named block of code, defined in the class, that performs an operation on or for the object; the methods define the object's behaviour.

Instantiation

Instantiation is the act of creating an object from a class. In most languages a constructor is called, often with the keyword new, which allocates memory for the new object and sets up its initial attribute values. Each instantiation produces a distinct object, independent of the others.

class Student
  attributes: name, yearGroup, mark
  methods: setMark(m), getGrade()

aoife = new Student("Aoife", 13)   // one object
ben   = new Student("Ben", 12)     // a separate object

Here aoife and ben are two objects of the same class. Calling aoife.setMark(78) changes only aoife's mark; ben is unaffected, because each object has its own copy of the attributes.

Worked example: modelling a real entity as a class

Examples in context

Example 1. A library system. A Book class defines attributes such as title, author and onLoan, and methods such as borrow() and returnItem(). Each physical book in the catalogue is one Book object. Borrowing one copy calls borrow() on that object and sets its onLoan attribute to true, leaving every other copy unchanged. The class is written once but models thousands of books.

Example 2. A game of characters. An Enemy class with attributes health and position and methods move() and takeDamage(amount) lets a designer create dozens of enemy objects from one definition. Each enemy tracks its own health, so damaging one does not heal another, and the shared methods give them consistent behaviour.

Try this

Q1. Define the term object and state how it relates to a class. [2 marks]

• Cue. An object is a specific instance of a class, created at run time with its own attribute values; the class is the template from which it is built.

Q2. A Dog class has attributes name and age and a method bark(). Write a statement that instantiates a Dog object called rex, and a statement that calls its bark() method. [2 marks]

• Cue. rex = new Dog("Rex", 3) then rex.bark().

Q3. Explain why grouping attributes and methods together in a class is described as modelling a real-world entity. [2 marks]

• Cue. The attributes capture what the thing knows (its state) and the methods capture what it can do (its behaviour), so the class mirrors a real object's data and actions in one unit.