Protected and private visibility in Ruby - Mini Project: Build & Apply

Understanding Protected and Private Visibility in Ruby

📖 Scenario: Imagine you are creating a simple Ruby class to represent a bank account. You want to control access to certain methods to protect sensitive information and actions.

🎯 Goal: You will build a Ruby class BankAccount that uses protected and private methods to control access to balance information and withdrawal actions.

📋 What You'll Learn

Create a class called BankAccount with an instance variable @balance

Add a protected method called display_balance that returns the balance

Add a private method called withdraw that subtracts an amount from the balance

Add a public method called transfer that uses withdraw and display_balance to transfer money between accounts

Print the balance of both accounts after the transfer

💡 Why This Matters

🌍 Real World

Controlling access to sensitive data and actions is important in real-world applications like banking software to prevent unauthorized access or changes.

💼 Career

Understanding method visibility is essential for writing secure and maintainable Ruby code, a skill valued in software development jobs.

1

Create the BankAccount class with initial balance

Create a class called BankAccount with an initialize method that sets an instance variable @balance to the value passed as a parameter.

Ruby

# Define class BankAccount
# Add initialize method with parameter amount
# Set @balance to amount
class BankAccount
  def initialize(amount)
    # Your code here
  end
end

Hint

Use def initialize(amount) and set @balance = amount.

2

Add a protected method to display balance

Inside the BankAccount class, add a protected method called display_balance that returns the value of @balance.

Ruby

class BankAccount
  def initialize(amount)
    @balance = amount
  end

  # Add protected method display_balance
  # Your code here
end

Hint

Use the protected keyword before defining def display_balance.

3

Add a private method to withdraw money

Inside the BankAccount class, add a private method called withdraw that takes an amount parameter and subtracts it from @balance.

Ruby

class BankAccount
  def initialize(amount)
    @balance = amount
  end

  protected

  def display_balance
    @balance
  end

  # Add private method withdraw(amount)
  # Your code here
end

Hint

Use the private keyword before defining def withdraw(amount).

4

Add a public transfer method and print balances

Add a public method called transfer that takes another BankAccount object and an amount. It should call withdraw(amount) on the current object and add the amount to the other account's @balance directly. Then, create two accounts, transfer 50 from the first to the second, and print both balances using display_balance.

Ruby

class BankAccount
  def initialize(amount)
    @balance = amount
  end

  protected

  def display_balance
    @balance
  end

  private

  def withdraw(amount)
    @balance -= amount
  end

  # Add public method transfer(other_account, amount)
  # Your code here
end

# Create two BankAccount objects
# Transfer 50 from first to second
# Print balances of both accounts
# Your code here

Hint

Use withdraw(amount) to reduce balance, other_account.display_balance to get the other balance, and instance_variable_set to add to it. Use send(:display_balance) to print balances.

Practice

1. Which statement best describes a private method in Ruby?

easy

A. It can be accessed from anywhere in the program.

B. It can only be called within the same object, not even by other instances of the same class.

C. It can be called by any object of the same class or its subclasses.

D. It is accessible only from outside the class.

Solution

1. Step 1: Understand private method access

   Private methods in Ruby are only accessible within the same object, meaning you cannot call them on other instances, even if they belong to the same class.

2. Step 2: Compare with other visibility levels

   Protected methods allow access from other instances of the same class or subclasses, but private methods do not.

3. Final Answer:

   It can only be called within the same object, not even by other instances of the same class. -> Option B

4. Quick Check:

   Private = same object only [OK]

Hint: Private = only inside same object, no other instances [OK]

Common Mistakes:

• Thinking private methods can be called on other instances
• Confusing private with protected visibility
• Assuming private methods are accessible outside the class

2. Which of the following is the correct syntax to declare a private method in a Ruby class?

easy

A. private
def method_name
end

B. private def method_name; end

C. def private method_name; end

D. def method_name; private; end

Solution

1. Step 1: Recall Ruby private method syntax

   In Ruby, you declare private methods by writing private on its own line, then defining the method below it.

2. Step 2: Check each option

   private on its own line followed by def method_name end is correct syntax. Putting private on the same line as def, prefixing def with private, or placing private inside the method are invalid.

3. Final Answer:

   private def method_name end -> Option A

4. Quick Check:

   private keyword on separate line before method [OK]

Hint: Use 'private' on its own line before method definition [OK]

Common Mistakes:

• Trying to put 'private' inside method definition line
• Using 'private def' which is invalid syntax
• Placing 'private' after method definition

3. What will be the output of this Ruby code?

class Person
  protected
  def secret
    "protected info"
  end

  public
  def reveal_secret(other)
    other.secret
  end
end

p1 = Person.new
p2 = Person.new
puts p1.reveal_secret(p2)

medium

A. SyntaxError

B. NoMethodError: protected method `secret' called

C. protected info

D. nil

Solution

1. Step 1: Understand protected method access

   Protected methods can be called by other instances of the same class. Here, p1 calls reveal_secret passing p2, and inside it calls other.secret.

2. Step 2: Predict output

   Since secret is protected, p1 can call secret on p2. The method returns "protected info", which is printed.

3. Final Answer:

   protected info -> Option C

4. Quick Check:

   Protected allows calls between same class instances [OK]

Hint: Protected allows access between same class instances [OK]

Common Mistakes:

• Thinking protected methods can't be called on other instances
• Expecting NoMethodError for protected method call
• Confusing protected with private access

4. Identify the error in this Ruby code:

class Animal
  private
  def sound
    "roar"
  end

  def make_sound(other)
    other.sound
  end
end

lion = Animal.new
tiger = Animal.new
puts lion.make_sound(tiger)

medium

A. NoMethodError: private method `sound' called for tiger

B. No error, outputs 'roar'

C. SyntaxError due to private method

D. ArgumentError in make_sound method

Solution

1. Step 1: Understand private method restrictions

   Private methods cannot be called with an explicit receiver, even if the receiver is an instance of the same class.

2. Step 2: Analyze the method call

   In make_sound, other.sound calls a private method on another instance, which is not allowed and raises NoMethodError.

3. Final Answer:

   NoMethodError: private method `sound' called for tiger -> Option A

4. Quick Check:

   Private methods can't be called on other instances [OK]

Hint: Private methods can't be called with explicit receiver [OK]

Common Mistakes:

• Assuming private methods can be called on other instances
• Confusing private and protected method rules
• Expecting no error when calling private method on other object

5. You want to allow instances of a class to call a method on each other but keep it hidden from outside calls. Which visibility should you use and how would you implement it?

class User
  # Your code here
  def compare_password(other)
    self.password == other.password
  end

  private
  def password
    "secret123"
  end
end

u1 = User.new
u2 = User.new
puts u1.compare_password(u2)

hard

A. Use attr_accessor for password and keep it private.

B. Make password a private method; this code works as is.

C. Make password public; private and protected won't work.

D. Make password a protected method; private won't allow other.password call.

Solution

1. Step 1: Understand method visibility for inter-instance calls

   Private methods cannot be called with an explicit receiver, so other.password will fail if password is private.

2. Step 2: Use protected visibility

   Protected methods allow calls between instances of the same class, so changing password to protected allows other.password to work while keeping it hidden from outside.

3. Final Answer:

   Make password a protected method; private won't allow other.password call. -> Option D

4. Quick Check:

   Protected allows inter-instance calls, private does not [OK]

Hint: Use protected for inter-instance method access [OK]

Common Mistakes:

• Keeping password private and expecting inter-instance calls to work
• Making password public and exposing sensitive data
• Using attr_accessor without protection