Protected and private visibility in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling a private method from outside the class

What will be the output of this Ruby code?

Ruby

class Person
  private
  def secret
    "my secret"
  end
end

p = Person.new
puts p.secret

A nil

B my secret

C NoMethodError

D SyntaxError

💡 Hint

Private methods cannot be called with an explicit receiver.

In Ruby, private methods cannot be called with an explicit receiver, even if that receiver is self. Calling p.secret raises NoMethodError.

❓ Predict Output

intermediate

Output of calling a protected method from another instance

What will this Ruby code print?

Ruby

class Person
  def initialize(name)
    @name = name
  end

  protected
  def secret
    "secret of #{@name}"
  end

  public
  def reveal_secret(other)
    other.secret
  end
end

p1 = Person.new("Alice")
p2 = Person.new("Bob")
puts p1.reveal_secret(p2)

A nil

B NoMethodError

C secret of Alice

D secret of Bob

💡 Hint

Protected methods can be called by instances of the same class.

Protected methods can be called by any instance of the defining class or its subclasses. p1 calls p2.secret successfully.

🔧 Debug

advanced

Why does this code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

class BankAccount
  private
  def balance
    1000
  end

  public
  def show_balance
    balance
  end
end

account = BankAccount.new
puts account.balance

A balance is private and cannot be called with an explicit receiver outside the class

B balance method is not defined

C show_balance method is private

D Syntax error in class definition

💡 Hint

Check where the private method is called from.

Calling account.balance from outside the class is not allowed because balance is private and called with an explicit receiver.

❓ Predict Output

advanced

Output when calling protected method from subclass instance

What will this Ruby code print?

Ruby

class Animal
  protected
  def sound
    "generic sound"
  end
end

class Dog < Animal
  def make_sound(other)
    other.sound
  end
end

d1 = Dog.new
d2 = Dog.new
puts d1.make_sound(d2)

A NoMethodError

B generic sound

C nil

D ArgumentError

💡 Hint

Protected methods are accessible within subclasses and instances of the same class.

Protected methods can be called by instances of the same class or subclasses. d1 calls d2.sound successfully.

📝 Syntax

expert

Identify the syntax error related to method visibility

Which option contains a syntax error in defining method visibility in Ruby?

A

class Test
  def secret
    42
  end
  private secret
end

B

class Test
  def secret
    42
  end
  private :secret
end

C

class Test
  private
  def secret
    42
  end
end

D

class Test
  private def secret
    42
  end
end

💡 Hint

Check how private is used to declare methods.

Option A is invalid syntax because 'private secret' is not a valid way to declare method visibility. It should be 'private :secret'.

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