Methods (OOP) (OCR A-Level Computer Science): Revision Notes

Methods (OOP)

Overview

In Object-Oriented Programming (OOP), methods are functions defined within a class that represent the behaviours of an object. They can manipulate an object's attributes and perform actions associated with the class. Like attributes, methods can be public or private, determining their accessibility within and outside of the class. Understanding methods, and the distinction between public and private methods, is essential for effective OOP.

Types of Methods

Public Methods

• Definition: Public methods are accessible from outside the class, allowing interaction with an object's functionality. These methods define the primary behaviours of the class that other parts of the programme can use.
• Purpose: Public methods serve as the primary interface of a class, allowing external code to interact with an object in a controlled way.
• Benefits:
  • Access and Interaction: Public methods provide controlled access to an object's functionality.
  • Encapsulation: They allow access to data and operations without exposing the internal workings of the class.

class Calculator:
    def add(self, a, b):
        return a + b  # Public method accessible from outside the class

calc = Calculator()
print(calc.add(3, 5))  # Outputs: 8

Private Methods

• Definition: Private methods are intended for internal use only, usually marked by a leading underscore (e.g., _method_name) in Python. These methods cannot be accessed directly from outside the class.
• Purpose: Private methods perform auxiliary or supportive tasks within the class. They often help break down complex processes and support the functionality of public methods without being exposed to the rest of the programme.
• Benefits:
  • Encapsulation and Security: Private methods help keep implementation details hidden, ensuring that only the necessary interface is exposed.
  • Modularity and Maintenance: Private methods can simplify complex tasks within the class, making code more modular and easier to maintain.

class BankAccount:
    def __init__(self, balance=0):
        self.balance = balance

    def deposit(self, amount):
        if amount > 0:
            self.balance += amount
            self._log_transaction(amount)  # Calls private method

    def _log_transaction(self, amount):  # Private method
        print(f"Transaction: Deposited ${amount}. New balance: ${self.balance}")

account = BankAccount()
account.deposit(100)  # Outputs: Transaction: Deposited $100. New balance: $100
# account._log_transaction(50)  # Error: Cannot directly access private method

Example: Bank Account Class with Public and Private Methods

class BankAccount:
    def __init__(self, account_holder, balance=0):
        self.account_holder = account_holder
        self.balance = balance

    # Public method for depositing money
    def deposit(self, amount):
        if amount > 0:
            self.balance += amount
            self._log_transaction("deposit", amount)

    # Public method for withdrawing money
    def withdraw(self, amount):
        if amount > 0 and amount <= self.balance:
            self.balance -= amount
            self._log_transaction("withdrawal", amount)
        else:
            print("Insufficient funds or invalid amount.")

    # Public method to check balance
    def check_balance(self):
        return self.balance

    # Private method to log transactions
    def _log_transaction(self, transaction_type, amount):
        print(f"{transaction_type.capitalise()} of ${amount}. New balance: ${self.balance}")

Key Points for Tracing and Writing Code with Methods

• Public Methods: These can be tested and called directly from outside the class. In the above example, deposit, withdraw, and check_balance provide controlled access to the bank account's functionality.
• Private Methods: Ensure that private methods are only used within the class. They provide essential internal operations, helping to keep the main logic of public methods clean and manageable.

Benefits of Using Public and Private Methods

1. Data Security: Private methods help protect the internal processes of the class, promoting encapsulation.
2. Code Readability: Public methods present a clear interface for external code, while private methods help organise complex processes within the class.
3. Modular Design: By segmenting complex actions into smaller, private methods, the class remains modular and easy to maintain.

Note Summary