Class

A blueprint for creating objects, often a feature of many object-oriented programming languages.

Attributes and variables

More or less an interchangeable term used to describe low-level storage slots attached to an object.

Properties

In most OO languages, properties are used to define getter and setter functions to change the value of or read and return the value of an attribute in a class. The way properties are defined varies across programming languages.

• C#

class SavingsAccount
{
	// Variables are made private to prevent illegal accessing out of the object
	private string accountNo;
	public string AccountNo
	{
		get { return accountNo; }
		set { accountNo = value; } // Can be extended to have additional checks or validation
	}
	
	private string accountName;
	public string AccountName { get; set; } // Shorthand
}

Methods

There are three types of methods in a user-defined class:

• constructors — used to create objects of a class;
• user-defined methods — used to perform a specific task related to the class; and
• printing methods — used to print out the class in a particular format when called by the programming language’s print function.

Constructors

A special function used to create objects of a class. Usually a required method in many OO programming languages.

The following code chunks below show some examples for different languages:

• C#

class SavingsAccount {
	// The name of the class and constructor must match.
	// Default constructor — does not have parameters
	public SavingsAccount() { }
	
	// Parameterised constructor with parameters
	public SavingsAccount(string no, string name, double bal)
	{
		AccountNo = no;
		AccountName = name;
		Balance = bal;
	}
}

• Python

class SavingsAccount:
	# The name of the constructor must be __init__.
	# Default constructor — does not have parameters
	def __init__(self):
		pass
	
	# Parameterised constructor with parameters
	def __init__(self, no, name, bal):
		self.accountNo = no
		self.accountName = name
		self.balance = bal

Printing methods

Because we won’t know what exactly to print out, we may need to define what to print out when a language’s printing function is called on an object of a class. The name of the function varies across programming languages.

• C#

class SavingsAccount {
	// The name of the function must be ToString.
	public override string ToString()
	{
		return "AccountNo: " + AccountNo +
			" AccountName: " + AccountName +
			" Balance: " + Balance;
	}
}

• Python

class SavingsAccount:
	# The name of the function must be __str__.
	def __str__(self):
		return "AccountNo: " + self.accountNo +
			" AccountName: " + self.accountName +
			" Balance: " + self.balance