Group Task

Finding patterns

Finding Patterns

In groups, determine a possible pattern that could be used to solve each of the following problems:

1. Sorting collections of records in different orders.
   • Strategy
2. Modelling a file system
   • Composite
3. Updating a UI component when the state of a program changes
   • Observer
4. Parsing and evaluating arithmetic expressions
   • Composite
5. Adjusting the brightness of a screen based on a light sensitivity
   • Observer

Composite Pattern

Composite Pattern

What type of design pattern is composite?

Composite Pattern

• No discrimination between a single (leaf) or a group (composite). Keeps code clean

Composite Pattern

Code Demo

Calculator.java - Composite pattern

Code Demo

Inside src/calculator, use the Composite Pattern to write a simple calculator that evaluates an expression. Your calculator should be able to:

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

Code Demo

{1 + [2 * (4 + 3)}

Factory Pattern

Factory Pattern

What type of design pattern?

Factory Pattern

1. The Product declares the interface, which is common to all objects that can be produced by the Creator and its subclasses.
2. Concrete products are different implementations of the product interface
3. The Creator class declares the factory method and returns new product objects
4. Concrete Creators override the base factory method so it returns a new type of product

Code Demo

Thrones.java - Factory Pattern

Code Demo

Inside src/thrones, there is some code to model a simple chess-like game. In this game different types of characters move around on a grid fighting each other. When one character moves into the square occupied by another they attack that character and inflict damage based on random chance. There are four types of characters:

• A king can move one square in any direction (including diagonally), and always causes 8 points of damage when attacking.
• A knight can move like a knight in chess (in an L shape), and has a 1 in 2 chance of inflicting 10 points of damage when attacking.
• A queen can move to any square in the same column, row or diagonal as she is currently on, and has a 1 in 3 chance of inflicting 12 points of damage or a 2 out of 3 chance of inflicting 6 points of damage.
• A troll can only move up, down, left or right, and has a 1 in 6 chance of inflicting 20 points of damage.

Code Demo

We want to refactor the code so that when the characters are created, they are put in a random location in a grid of length 5.

1. How does the Factory Pattern (AKA Factory Method) allow us to abstract construction of objects, and how will it improve our design with this new requirement?
   • Abstract the construction of the character objects. We don't deal with the constructor, instead call a general factory method that handles the number.
2. Use the Factory Pattern to create a series of object factories for each of the character types, and change the main method of Game.java to use these factories.