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'.