COMP2511

Tutorial 7

Overview

  • Assignment 2 tips
  • Composite Pattern 
  • Factory Pattern
  • Decorator Pattern

Good & Bad News

  • Congrats on finishing assignment 1!
    • Marks and feedback will be coming soon!
  • Assignment 2 should be underway (sorry 🙇)
    • Incorporate feedback from assignment 1

Assignment 2 Tips

Assignment 2 Tips

Blog Posts

  • Follow the templates in the spec and attach the merge request to the correct task
  • Tutors will not have enough time to look through your codebase for things not mentioned in the blogs
  • Make it easy for us to award you marks :D 

Assignment 2 Tips

Task 1 Part f)

  • Address all 3 problems mentioned in the spec
  • Find and resolve more problems for higher marks
  • Do not intentionally write bad code in earlier parts and refactor them in this section
  • Do not refactor to resolve a problem that does not exist yet

Code Smells

If you think you have found a code smell, make sure to double check here: Code Smells (refactoring.guru)

Assignment 2 Tips

Task 2

  • All groups must complete part a
  • Choose N tasks from b-f, where N is the number of members in your group
  • Earlier subtasks are easier but doing them will not result in a high mark
    • We give you the flexibility to decide what a desirable grade is to your group

Composite Pattern

Composite Pattern

Suppose we have

  • A File can store data like text or images etc
  • A Directory can store both files and other directories

Composite Pattern

Suppose we have

  • A File can store data like text or images etc
  • A Directory can store both files and other directories
  • Together files and directories form a hierarchy that makes up a FileSystem

This is a tree-like structure!

Composite Pattern

UML Diagram

  •  File (Leaf) and Directory (Composite) both implement a common interface
    • ​They can be treated as the "same thing"
  • A Directory can store both files and directories (i.e. add and remove)

Composite Pattern

Use the composite pattern to design a simple calculator that can be used to evaluate an arithmetic expression

Code Demo

Your calculator should be able to:

  • Add two expressions
  • Subtract two expressions
  • Multiply two expressions
  • Divide two expressions

Composite Pattern

Types of expressions

Goal: have one function that works for both

Calculator.calculate(exp1);
Calculator.calculate(exp2);
Expression exp1 = 42;

Expression exp2 = ((1 + 2) - ((3 / 4) * 5);

Composite Pattern

Tree representation

Expression exp1 = 42;

Just a Leaf containing the value

Composite Pattern

Tree representation 

Expression exp2 = ((1 + 2) - ((3 / 4) * 5);
Expression exp2 = ((1 + 2) - ((3 / 4) * 5);

Composite Pattern

Tree representation 

Expression exp2 = ((1 + 2) - ((3 / 4) * 5);

Composite Pattern

Tree representation 

Expression exp2 = ((1 + 2) - ((3 / 4) * 5);

Composite Pattern

Tree representation 

Expression exp2 = ((1 + 2) - ((3 / 4) * 5);

Composite Pattern

What is it?

Model a family of classes in a tree-like hierarchy to represent the part-whole relationship

Structural design pattern that allow a group of objects to be treated like single instances of the object

  • Single instance: File
  • Group of objects: Directory

In Unix operating systems, a Directory is a File!

When to use?

Factory Pattern

Factory Pattern

Suppose we have

  • An ImageViewer application that lets users delete photos from their mobile device
  • We need a separate delete Button for each OS
    • IOS
    • Android
  • We want to avoid code like this nn a constructor:
    • if OS == IOS then create <IOSButton>
      else create <AndroidButton>
    • Problematic if there are more type of buttons with different conditions of initialisation

Example adapted from Fireship.io

Factory Pattern

Inside src/thrones, there is some code to model a simple chess-like game. The game has different types of characters that can move and attack in different ways

For this week, the implementation logic of the game does not matter

Instead, we focus on the creation logic of these character classes!

Code Demo

Factory Pattern

Refactor the code such that when the characters are created, they are put in a random location in a grid of length of length 5

What would be the main benefit of using the factory pattern?

Code Demo

The business logic of constructing characters are completely hidden from client code

  • They don't need to know about the grid of length 5
  • They won't know even if we change the logic (i.e. grid of length 7)

Factory Pattern

Factory Pattern

Why use it?

What is it?

Creational design pattern that lets you abstract (hide away) the business logic (i.e. checking the OS) of constructing an object

  • Reduce coupling between client code and the specific classes being instantiated
    • Users don't need to know how objects are created (they simply call a method from the factory)
  • Follows the single responsibility principle
    • Construction of every class in the family is handled at the same place

Decorator Pattern

Decorator Pattern

Suppose we have:

  • A Human class that can wear clothes to boost their fashion, warmth, etc
  • What OOP technique can we use to represent a Human wearing a sweater and coat?
    • Inheritance: create a new sub class SweaterCoatHuman that extends from Human
  • What if a Human object can wear a combination of any clothes or change their clothes at runtime?
    • Can't keep making subclasses... there are infinite combinations
    • Inheritance is also static so we can't add clothing to a Human at runtime

Decorator Pattern

UML Diagram

  • Human is what we want to decorate 
  • The HumanDecorator implements the Human interface and composes Human
  • Decorated Human and a regular Human can be treated same by client code
  • Implementation of doSomething in the decorator classes can now "enhance" the implementation of its wrappee 

Decorator Pattern

Code Demo

Let's try implementing this in Java!

Decorator Pattern

What is it?

Structural design pattern that allows the behaviour of objects to be enhanced at runtime by placing them into special wrapper objects

What problem does it solve?

Enhance the behaviour of an object at runtime without violating open-closed principle

The End

COMP2511 Tutorial 7

By matthewliu-2

Source: Matthew Liu

COMP2511 Tutorial 7

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