Open classes (reopening classes) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of reopening a class and adding a method?

Consider the following Ruby code that reopens the String class to add a new method. What will be printed when the code runs?

Ruby

class String
  def shout
    self.upcase + "!"
  end
end

puts "hello".shout

A HELLO!

B hello!

C Error: undefined method 'shout' for String

D hello

💡 Hint

Reopening a class allows you to add new methods to existing classes.

The String class is reopened and a new method shout is added. Calling shout on "hello" returns the uppercase version plus an exclamation mark.

❓ Predict Output

intermediate

What happens when you redefine an existing method in a reopened class?

Look at this Ruby code where the Array class is reopened and the push method is redefined. What will be the output?

Ruby

class Array
  def push(element)
    "Cannot add elements"
  end
end

arr = [1, 2]
puts arr.push(3)

A Error: wrong number of arguments for push

B "Cannot add elements"

C [1, 2, 3]

D 3

💡 Hint

Redefining a method replaces the original behavior.

The push method is redefined to always return the string "Cannot add elements" instead of adding to the array.

🔧 Debug

advanced

Why does this reopened class code raise an error?

This code reopens the Integer class to add a method, but it raises an error. What is the cause?

Ruby

class Integer
  def double
    self * 2
  end
end

puts 5.double

class Integer
  def double
    self + 2
  end
end

puts 5.double

A Error because Integer is a built-in class and cannot be reopened

B Error because method double is defined twice without aliasing

C No error; outputs 10 then 7

D Error because puts is called outside any method

💡 Hint

Ruby allows reopening built-in classes and redefining methods.

Ruby allows reopening Integer and redefining methods. The first puts prints 10 (5*2), the second prints 7 (5+2). No error occurs.

🧠 Conceptual

advanced

What is a risk of reopening core Ruby classes?

Reopening core Ruby classes like String or Array can be powerful but risky. Which of these is the main risk?

A It causes Ruby to crash immediately

B It makes the program run slower because classes are duplicated

C It prevents you from using built-in methods anymore

D It can cause conflicts if multiple libraries redefine the same method differently

💡 Hint

Think about what happens if two pieces of code change the same method.

Reopening core classes and redefining methods can cause conflicts and unexpected behavior if different code changes the same method in incompatible ways.

❓ Predict Output

expert

What is the output after reopening a class and using super?

Examine this Ruby code where the String class is reopened and the reverse method is redefined using super. What will be printed?

Ruby

class String
  def reverse
    "Reversed: " + super
  end
end

puts "abc".reverse

A Reversed: cba

B abc

C Error: super called outside method

D Reversed: abc

💡 Hint

super calls the original method from the class before reopening.

The new reverse method calls super to get the original reversed string, then adds "Reversed: " before it.