Ruby Interview Questions and Answers

A Symbol is an immutable, interned identifier such as :name. A String is a mutable sequence of characters such as "name".

Symbols are stored once, so repeated use is memory-efficient and comparison is by identity. That makes them ideal as hash keys, state names and method names. Strings are for text you display or manipulate, and each literal can create a new object.

:name.object_id == :name.object_id    # true, same object
"name".object_id == "name".object_id  # false, different objects

h = { name: "Ada" }                   # symbol key

Since Ruby 2.2 symbols are garbage-collected, but converting untrusted input with to_sym can still be a memory concern, so avoid it on unvalidated user data. Convert with to_sym and to_s. Prefer frozen string literals reduce allocations for repeated strings. Use symbols for identifiers, strings for data.

Ruby encapsulates instance variables such as @name, which are private by default. attr_reader, attr_writer and attr_accessor are class macros that define getter and setter methods.

class User
  attr_reader   :id       # def id; @id; end
  attr_writer   :email    # def email=(value); @email = value; end
  attr_accessor :name     # both getter and setter
end

u = User.new
u.name = "Ada"
puts u.name

attr_accessor :a, :b defines several at once. These are ordinary methods, so they can be overridden or made private with private :name=, restricting external writes.

attr_* accepts an optional boolean second argument to skip instance-variable initialisation warnings. Using accessors internally rather than @name directly lets subclasses override behaviour and keeps the interface consistent. For deeply immutable objects use attr_reader plus freeze.