COMP2511

Tutorial 10

Today

  • Template Pattern
  • Visitor Pattern
  • Kahoot

Reminders

If you did assignment-iii, please make sure you submit it following the instructions in the spec

  • No autogenerated UMLs
  • No ChatGPT blogs
  • Make the video accessible to the public
    • We will not be chasing this up since marking will happen during week 11/12

By Alvin

Template Pattern

Template Pattern

Suppose:

  • I created a ResumeTemplate and gave it to everyone in the class
  • You can edit the content within each section of the template to match your personal information
  • But you cannot edit the structure of the template itself

 

Think of templates from MS word

Template Pattern

UML Diagram

  • generateResume is the template method that can be split into smaller "steps"
  • Steps can be overridden by subclasses and they are the hook methods
  • Methods that have default implementation and cannot be overridden are final methods
    • Why is ​generateResume final?

Template Pattern

Behavioural design pattern that lets you

  • Define the skeleton (i.e. steps) of an algorithm in a super class
  • Override specific steps of the algorithm in subclasses without modifying the overall structure of the algorithm

Why use it?

  • Subclasses do not need to duplicate the entire algorithm each time 
  • By only allowing subclasses to modify hook methods, they cannot accidentally break the algorithm 

What is it?

Template Pattern

Code Demo

Model how characters in a game can move around and attack other characters

  • A King can
    • Move one square in any direction (including diagonally)
    • Always deal 8 points of damage when attacking
  • A Queen can
    • Move to any square in the same column, row or diagonal as her current position
    • Deal 12 points of damage with a 1 in 3 chance or 6 points of damage otherwise
  • A Dragon can
    • Move up, down, left or right
    • Inflict 20 points of damage with a 1 in 6 change

Visitor Pattern

Visitor Pattern

Suppose:

  • I have different types of characters in a Game
  • If I want to allow users to save the game, I need to be able store information about the game onto disk 
  • Convert information from the game into JSON files
  • But how can I convert instances of some class into a JSON file?

Add an interface JSONExport and implement it for every class that can be exported 

Visitor Pattern

What if now I can't edit the source code of these classes directly?

  • The game could be a legacy system, so the original authors don't want anyone to change the code anymore
  • The game could be developed by a different team in the company and they also they want other people messing with their code

Visitor Pattern

  • Visitable: classes that need the new feature
    • Must have an accept method that takes a visitor
  • Visitor: classes that will add a new feature to the visitable
    • Must have a visit method for each type of visitable

UML Diagram

Visitor Pattern

Code Demo

Visitor Pattern

What is it?

Benefits​

Behavioural design pattern that allow functionalities to be added to classes without needing to "open" them

​When to use?

  • Obey the Open-close principle 
  • Obey the Single responsibility principle because every new functionality is a new "visitor"

New operations need to be added to all elements of an existing object structure

Kahoot

kahoot

The End

The End

Thank you all for a great term and good luck for your exams 🎉

COMP2511 Tutorial 10

By matthewliu-2

Source: Matthew Liu

COMP2511 Tutorial 10

  • 55

More from matthewliu-2