class Test private
 def test_print
  puts 'test'
 end
end

class Test2 < Test
 def test_print2
  # self.test_print #=> 这里加上self就不能调用，private method `test_print' called for # (NoMethodError)
  test_print #=> 不加self就能调用
 end
end

Test2.new.test_print2