Week 05 Tutorial
H12A
Strategy
Observer
State
Design Patterns: The Origin Story
- The concept of patterns was first described by Christopher Alexander in A Pattern Language: Towns, Buildings, Construction.
Design Patterns: The Origin Story
- The idea was picked up by four authors: Erich Gamma, John Vlissides, Ralph Johnson, and Richard Helm. In 1994, they published Design Patterns: Elements of Reusable Object-Oriented Software, in which they applied the concept of design patterns to programming.
The Gang of Four
Strategy Pattern
Key Characteristics
- Defines a family of algorithms
- Encapsulates each algorithm
- Allows algorithms to be interchangeable
private SortStrategy strategy;
BubbleSort
BubbleSort
MergeSort
QuickSort
HeapSort
Strategies
DataSorter
<<SortStrategy>> {sort()}
private SortStrategy strategy;
strategy.sort();
void setStrategy (SortStrategy);
Strategy Pattern
When to Use
- You need different variants of an algorithm
- You want to avoid multiple conditionals in your code.
- You have a set of related classes that differ only in their behaviour
State Pattern
Key Characteristics:
- Allows an object to change its behaviour when its internal state changes
- Encapsulates state-specific behaviour in separate classes
- Transitions between states are typically managed by the state classes themselves
State Pattern
When to Use
- An object’s behaviour depends on its state, and it must change its behaviour at runtime depending on that state
- Operations have large, multipart conditional statements that depend on the object’s state
The crucial difference
While both patterns involve changing an object’s behavior, the key difference lies in their intent:
- Strategy focuses on making algorithms interchangeable.
- State focuses on allowing an object to change its behaviour when its internal state changes.
Breaking LSP 🐣
-
Give an example of a real-life subclass that would violate the given contract:
List<String> singers = List.of("Taylor Swift", "Jimi Hendrix", "Beyoncé", "Tyler the Creator", "Adele");
List<String> lineup = singers.stream()
.filter(s -> hasGrammy(s)) // 🎤 Filter Grammy winners
.map(s -> styleOutfit(s)) // 💄 Give stage outfit
.toList(); // 🚌 Collect them for their performance
Note: by real-life subclass, I mean some kind of creature that is a type of Bird.
Observer Pattern
Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, its dependent objects are to be notified automatically.