JavaRubyInheritenceComparison.rb

class A
  def methoda_public
    puts "Hello from methoda_public self=#{self}"
  end

  def methoda_private
    puts "Hello from methoda_private self=#{self}"
  end

  def methoda_protected
    puts "Hello from methoda_protected self=#{self}"
  end

    # 1) Java => class methods are defined using static keyword. instances can access class methods in Java.
    # 2) Ruby => class methods are actually singleton methods of class itself. (in Ruby a class is an object itself.). class methods are actually defined in
               # singleton class of "class object" and hence called as singleton methods. Only that object can call singleton method on which it is defined and none other.
               # Singleton methods are Exclusive to a single particular object only. Hence even the instances created from class A cannot access class methods.
               # derived class can invoke base class' singleton object. Again instances of Derived class cannot access singleton method defined on base class.
  def self.methoda_classmethod_public
    puts "hello from methoda_classmethod_public self=#{self}"
  end

  private :methoda_private
  protected :methoda_protected
end

class B < A
  def public_methodb_calling_private_methoda

      # 1) Java => private members are not inherited directly in derived class. So this will not work in Java.
                 # Derived class inherits base class' protected and public members. So if Base class' protected/public method access private member
                 # then Derived class can also 'work' on private member via inherited protected/public methods.
      # 2) Ruby => private members are inherited in Ruby. So this will work in ruby.
    methoda_private       

      # 1) Java => you cannot access private memebers directly in derived class. So this will not work in Java.
      # 2) Ruby => Will not work as private members must be accessed WITHOUT explicit receiver in derived class.
    #A.new.methoda_private  # A new object is created and we are trying to access its private member.  will not work.
    #self.methoda_private   # An equivalent in Java would be this.methoda_private(). But ofcourse in Java private members are not inherited so will not work.
      
      # 1) Java => Protected members can be accessed in derived class directly becuase they are inherited. So this will work.
      # 2) Ruby => Protected members are inherited in Ruby as well. So this will work.
    methoda_protected

      # 1) Java => protected memeber cannot be accessed in drived class using reference variable(i.e. by creating object).
      # 2) Ruby => This will work. But this will NOT work in Ruby also if below expression i.e. A.new.methoda_protected is called outside class B. 
    A.new.methoda_protected

      # 1) Java => A Java equivalent would be this.methoda_protected() . This will work in Java. As protected methods are inherited in Java directly. Needed verification.
      # 2) Ruby => This will work. Ruby will be calling inherited method. Java will also do the same.
    self.methoda_protected

      # 1) Java => Instance method can call static method (class method).
      # 2) Ruby => This will not work. Instance method CANNOT call class method.
    #methoda_classmethod_public

      # 1) Java => instance method can call static method (class method). In java it would be this.methoda_classmethod_public()
      # 2) Ruby => this will not work. Because inside instance method self will be referring to the instance of B. And instance of B CANNOT call
                 # singleton method of class A.
    #self.methoda_classmethod_public
  end

    # 1) Java => Both below statements are not allowed in Java. These are called outside any method definition.
    # 2) Ruby => Both below will work in Ruby. Hence class methods are inherited but only to be called by derived class itself.
               # class methods are singleton method. Normally singleton methods are called by only their own onjects.
               # But when in case of singleton methods defined on class (class methods), they can be called by dervied class also. (Not by derived class instances).
  methoda_classmethod_public
  self.methoda_classmethod_public
end

#A.new.methoda_private     # => will not work. (same as in Java)
#A.new.methoda_protected   # => will not work. (same as in Java). Compare and take a note of this expression inside class B as well.
#A.new.methoda_classmethod_public # => Will not work in Ruby but will work in Java. Java syntax would be: A a = new A(); a.methoda_classmethod_public() .

B.new.public_methodb_calling_private_methoda
B.methoda_classmethod_public  # => will work both in Java and Ruby