SRP: Single Responsibility 

A class should have a single responsiblity

OCP: Open-Closed

Software entities should be open for extension but closed for modification

LSP: Liskov Substitution

objects in a program should be replaceable with instances of their subtype without altering correctness of the program.

ISP: Interface segregation

Many client-specific interfaces are better than one general-purpose interface

DIP: Dependency Inversion

one should depend upon Abstractions.

  #ruby
  puts 'What is your name?'
  name = gets
  process_name(name)
  puts 'What is your quest?'
  quest = gets
  process_quest(quest)

require 'tk'
  root = TkRoot.new()
  name_label = TkLabel.new() {text "What is Your Name?"}
  name_label.pack
  name = TkEntry.new(root).pack
  name.bind("FocusOut") {process_name(name)}
  quest_label = TkLabel.new() {text "What is Your Quest?"}
  quest_label.pack
  quest = TkEntry.new(root).pack
  quest.bind("FocusOut") {process_quest(quest)}
  Tk.mainloop()

• ​​Software Design Pattern
• Implements IOC
• A dependency is an Object ( service)
• Assembler