Rubyのクラスメソッドとインスタンスメソッド 仕組みとベストプラクティス

class Example
  def self.class_method
    "This is a class method"
  end

  def instance_method
    "This is an instance method"
  end
end

puts Example.class_method        # => "This is a class method"
puts Example.new.instance_method # => "This is an instance method"

class Example
  # selfを使った定義
  def self.method_one
    "Class method one"
  end

  # 特異クラスに直接定義
  class << self
    def method_two
      "Class method two"
    end
  end
end

puts Example.method_one # => "Class method one"
puts Example.method_two # => "Class method two"

class Example
  def greet(name)
    "Hello, #{name}!"
  end
end

example = Example.new
puts example.greet("Ruby") # => "Hello, Ruby!"

class MathUtility
  def self.square(number)
    number ** 2
  end
end

puts MathUtility.square(4) # => 16

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

  def greet
    "Hi, I'm #{@name}"
  end
end

person = Person.new("Alice")
puts person.greet # => "Hi, I'm Alice"

class Counter
  @@count = 0

  def initialize
    @@count += 1
  end

  def self.total_count
    @@count
  end
end

Counter.new
Counter.new
puts Counter.total_count # => 2

class Configuration
  @setting = "default"

  def self.setting
    @setting
  end

  def self.setting=(value)
    @setting = value
  end
end

puts Configuration.setting # => "default"
Configuration.setting = "custom"
puts Configuration.setting # => "custom"

class Person
  def self.create_with_name(name)
    new(name)
  end

  def initialize(name)
    @name = name
  end

  def greet
    "Hello, #{@name}"
  end
end

person = Person.create_with_name("Bob")
puts person.greet # => "Hello, Bob"

class Example
  def self.show_self
    self
  end
end

puts Example.show_self # => Example

class StringUtility
  def self.capitalize_words(string)
    string.split.map(&:capitalize).join(" ")
  end
end

puts StringUtility.capitalize_words("hello world") # => "Hello World"