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Example: the singleton pattern

From: Foundations of Programming: Object-Oriented Design

Video: Example: the singleton pattern

I'm going to go through two design patterns and illustrate them as simply as possible, one with code and one without. So you both see an implementation of one, and you get the general idea that this same Design Pattern could be done in multiple programming languages. But it's always the idea, the approach, that matters. Now first up is the Singleton Design Pattern and here's the problem it's designed to solve. We create a class, and we know the classes can be instantiated once or twice or a thousand times, but what if you only want one object of that class.

Example: the singleton pattern

I'm going to go through two design patterns and illustrate them as simply as possible, one with code and one without. So you both see an implementation of one, and you get the general idea that this same Design Pattern could be done in multiple programming languages. But it's always the idea, the approach, that matters. Now first up is the Singleton Design Pattern and here's the problem it's designed to solve. We create a class, and we know the classes can be instantiated once or twice or a thousand times, but what if you only want one object of that class.

What if you want to restrict this. In fact, you need there to be only one object of this class? What if this one object needs to represent the currently running application or perhaps the current display and having more than one instantiated object just doesn't make sense and could even lead to problems with the application. There is actually quite a lot of situations where there should only be one object of a particular class. Now this might first sound a little like an Abstract Class, but remember an Abstract Class isn't allowed to be instantiated at all.

So, if we have only one, well, you might first think why don't we just not create more than one? And you could do that, but it's not really enforcing anything, you can't guarantee that behavior. So instead, we can use the Singleton Design Pattern. With the Singleton Design Pattern we ensure that a class only has one instance, and we provide just one way to get to it. Meaning, we don't want to have to write code to instantiate the Singleton, we want to assume that it always exists, there is always one of them, and we just want to be able to ask for it from any other point of the program and get it.

Now, how this is implemented can vary across languages, but what I am going to demonstrate is the classic technique as used in Java. So first we create a class. It's a regular public class. I've called it MySingleton. And this class can contain methods and instance variables and all the normal stuff, there is nothing different about it yet. But because there's nothing different, this class could be instantiated a thousand times, and we need that not to be true. So we actually add a constructor, and I mark it as private.

And what this means is nobody can actually instantiate the class from outside, nobody can just say they want to create a new MySingleton object. However, that doesn't really solve our problem that we need there to be one of them. We just can't create it from the outside. So, how does this object actually come into being? Well, I have two things to do here. What I am first going to do is create a static variable in this class that holds a placeholder to a singleton object, I've called it __me and set it equal to null.

This just means a variable that can hold a singleton object, and there is nothing in it right now. And then we create a new method, and this is the important part of the Singleton Design Pattern. We could call this method anything we wanted to, getInstance is pretty common. And it's a static method, meaning it's called on the class itself. We don't have to have an instance of this class yet. The first thing it asks is do I exist? Is there anything in that __me variable, if it's null then instantiate a new MySingleton object.

Now we're allowed to do that from inside this class, because we marked that constructor as private, which means this class is allowed to call it but nobody else is. And then after the if statement, we'll return that object. And what this means is that it's the getInstance static method of the MySingleton class that is the important way to get to this singleton object. We are actually using a technique here called lazy instantiation, which means that until someone asks for this object it doesn't exist. But when they ask for it, we'll look for it.

If it doesn't exist, we create it, we store a reference to it so that when the next person asks it's already there. So the code that I've just added is all we need, and we've completely turned this into a singleton. As far as anyone else is concerned, there is only one of these. There'll always be one of these. Now the way we would ask for it in another part of the application is just by saying MySingleton.getInstance. That will call that method, check to see if the object exists, if it doesn't, it will be instantiated once and then returned.

If it does exist, it will just be returned. We can then start using the methods of that singleton. In fact, you could really combine this and just call it directly and anywhere else in the application can use this method to get to it. And that is the Singleton Design Pattern. It might seem a little convoluted, but bear in mind it's just a few lines of code, and this works very well. There'll never be any more than one of these objects that's accessible from anywhere in the application without ever having to manually instantiate it.

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This video is part of

Image for Foundations of Programming: Object-Oriented Design
Foundations of Programming: Object-Oriented Design

47 video lessons · 47496 viewers

Simon Allardice
Author

 
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  1. 11m 35s
    1. Welcome
      1m 25s
    2. Who this course is for
      1m 15s
    3. What to expect from this course
      3m 6s
    4. Exploring object-oriented analysis, design, and development
      1m 41s
    5. Reviewing software development methodologies
      4m 8s
  2. 26m 14s
    1. Why we use object-orientation
      2m 42s
    2. What is an object?
      5m 22s
    3. What is a class?
      4m 43s
    4. What is abstraction?
      2m 45s
    5. What is encapsulation?
      3m 45s
    6. What is inheritance?
      3m 35s
    7. What is polymorphism?
      3m 22s
  3. 12m 16s
    1. Understanding the object-oriented analysis and design processes
      4m 13s
    2. Defining requirements
      6m 9s
    3. Introduction to the Unified Modeling Language (UML)
      1m 54s
  4. 23m 35s
    1. Understanding use cases
      6m 11s
    2. Identifying the actors
      4m 16s
    3. Identifying the scenarios
      5m 7s
    4. Diagramming use cases
      4m 18s
    5. Employing user stories
      3m 43s
  5. 16m 36s
    1. Creating a conceptual model
      1m 59s
    2. Identifying the classes
      2m 27s
    3. Identifying class relationships
      2m 38s
    4. Identifying class responsibilities
      6m 43s
    5. Using CRC cards
      2m 49s
  6. 22m 25s
    1. Creating class diagrams
      6m 11s
    2. Converting class diagrams to code
      4m 57s
    3. Exploring object lifetime
      5m 55s
    4. Using static or shared members
      5m 22s
  7. 19m 49s
    1. Identifying inheritance situations
      6m 49s
    2. Using inheritance
      2m 43s
    3. Using abstract classes
      2m 2s
    4. Using interfaces
      4m 20s
    5. Using aggregation and composition
      3m 55s
  8. 9m 23s
    1. Creating sequence diagrams
      5m 18s
    2. Working with advanced UML diagrams
      2m 3s
    3. Using UML tools
      2m 2s
  9. 10m 39s
    1. Introduction to design patterns
      2m 40s
    2. Example: the singleton pattern
      4m 53s
    3. Example: the memento pattern
      3m 6s
  10. 21m 47s
    1. Introduction to object-oriented design principles
      2m 50s
    2. Exploring general development principles
      3m 55s
    3. Introduction to SOLID principles
      6m 43s
    4. Introduction to GRASP principles
      8m 19s
  11. 7m 1s
    1. Reviewing feature support across different object-oriented languages
      3m 50s
    2. Additional resources
      2m 27s
    3. Goodbye
      44s

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