CPSC 210
D6: Singleton Pattern
Learning Goals
- (To review static fields and static methods)
- To apply the Singleton Design Pattern to a given problem
Recap: Static Fields & Methods
- A static field is associated with the class rather than with an instance of a class.
- As a result, every instance of the class shares a variable that is declared static.
- As a result, every instance of the class shares a variable that is declared static.
- Static fields and methods are fundamental to the
implementation of the Singleton pattern
Recap: Static Fields & Methods
- A static method has access only to those fields that are declared static. It cannot access instance data.
- As a result, a static method can be invoked using the name of the class.
Order
Motivation
I want to coordinate a state across my system. For this purpose, I create a class for my state.
How do I make sure there is always only one instance of my State class in my system?
public class WifiState {
private int numberOfClients;
private int mbDownloadVolume;
private int mbUploadVolume;
public WifiState() {}
// setters
}// ...somewhere in code...
// State state = new WifiState();
// state.setNumberOfClients(30);
// ...// ...somewhere in code...
// How do I get the current state?The Singleton Pattern
Gang of Four: "Ensure a class only has one instance, and provide a global point of access to it."
Wikipedia: "Design pattern that restricts the instantiation of a class to one 'single' instance. This is useful when exactly one object is needed to coordinate actions across the system."
The Singleton Pattern
The Singleton Pattern (2)
public class WifiState {
private int numberOfClients;
private int mbDownloadVolume;
private int mbUploadVolume;
public WifiState() {}
// setters
}public class WifiState {
private static WifiState INSTANCE;
private int numClients = 0;
private int mbDownloadRate = 0;
private int mbUploadRate = 0;
private WifiState() {}
public static WifiState getInstance() {
if (INSTANCE == null) {
INSTANCE = new WifiState();
}
return INSTANCE;
}
}Why not just static instead?
- We could also implement a global state as a static class
/**
* Represents a log of alarm system events.
* We use the Singleton Design Pattern to ensure that there is only
* one EventLog in the system and that the system has global access
* to the single instance of the EventLog.
*/
public class EventLog implements Iterable<Event> {Wait...where have I seen this?
- Have we not already seen examples on this...?
- Am I not working with a Singleton right now...?
- Indeed! Project Phase 4!
Let's have a look...
Problems with Singleton
- The Singleton pattern breaks OO conventions
- No abstraction or polymorphism introduced
- Rather, introduces a new global method
- Singleton itself breaks SRP:
- Resp. 1: maintain only one instance
- Resp. 2: whatever else the class is doing
- Nowadays often superseded by "dependency injection":
- Singleton instances are "provided" to you by a framework and you don't use the pattern yourself
- If you want to learn more, take CPSC 310 😇
Lecture Lab
Highlander (THE ONE HIGHLANDER):
/>___________________________________
[########[]_________________________________/
/>
Highlander (THE ONE HIGHLANDER):
/>___________________________________
[########[]_________________________________/
/>Desired Output
Highlander (Joe's Highlander):
/>___________________________________
[########[]_________________________________/
/>
Highlander (Sue's Highlander):
/>___________________________________
[########[]_________________________________/
/>Actual Output
- Joe and Sue are playing the Highlander game
- But there are multiple Highlanders
Lecture Lab
