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Design Patterns: Singleton in Java

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13th Nov, 2021
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Design Patterns: Singleton in Java

Design Patterns are beneficial for programmers when they need to utilize the best available practices in object-oriented software development. Design Patterns can be understood as well-proved and tried and tested solutions for solving the design problem at hand. Design Systems describes the problem, the solution, when to apply the solution, and its consequences. In a way, Design Patterns can be thought of as different templates that software developers can implement when they are solving specific object-oriented design problems. 

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Some of the features of using Design patterns in Java as compared to coding from scratch include: 

  • Design Patterns are already defined and implemented. As a result, they provide a standard approach to solve repetitive problems. In that sense, it helps programmers save a lot of time and effort. 
  • Using Design Patterns ensures the reusability of your piece of code, which leads to high maintainability and robustness of your Java code. All of this then reduces the total cost of ownership (TCO) of the software at hand. 
  • Design Patterns make the code easy to read and understand, which leads to faster communication and seamless software development. 

Java programming language offers various Design Patterns for programmers and developers, which can broadly be broken down into three categories: 

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  • Creational Design Patterns
  • Structural Design Patterns
  • Behavioural Design Patterns

All of these patterns are used for different use cases and purposes. Through this article, let’s talk about one kind of Design Pattern in Java – Singleton Design Pattern, which falls under the Creational Design Patterns category. 

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Singleton Design Pattern in Java

Singleton Design Pattern belongs to the Creational Design Pattern category. It ensures that a class only has one instance, providing this instance with a global access point. In doing so, the Singleton Design Pattern solves two problems at once – these include: 

1. It ensures that a given class has only one instance. 

This is important to consider, especially when working on shared resources, like a database or a file. Imagine that you created an object, but after a while, decided to create a new one. In the Singleton Design Pattern, instead of receiving a new object, you’ll get the one you already made. That way, this Design Pattern ensures no redundancy of objects and that a class has only one instance. This behaviour is impossible to replicate with constructors because constructor calls must always return a new object. 

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2. Provides a global access point to the particular instance.

The global variables that you use to store essential objects are pretty handy. They are also equally unsafe since any piece of code has the power to overwrite the contents of your global variables, leading to an erroneous piece of code for you. Just like global variables, Singleton Patterns allows you to access different objects from anywhere in the program. However, unlike global variables, Singleton Patterns secures that instance from being overwritten by another program.

Nowadays, the Singleton Design Pattern is so widely used and prevalent that programmers may call something Singleton even if it solves just one of the above-listed problems. All the implementations of Singleton Design Patterns have the following two steps in common: 

  • Making the default constructor private to prevent other classes and objects from using new operators with Singleton classes. 
  • Building a static creation method to function as a constructor. This method, then, calls the private constructor to create an object and saves it in a static field. All the upcoming calls to this particular method now return the cached object. 

The government’s functioning is an excellent real-life example to understand how Singleton Design Patterns work. Regardless of the personal identities of the individuals who constitute the governments, the title, “The Government of X”, is the global point of access that identifies the group of people being talked about.

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Implementing Singleton Design Pattern in Java

Please go through the following list of steps to implement your own Singleton Design Pattern in Java: 

  • Add a private static field to the class where you wish to store the singleton instance. 
  • To get the appropriate singleton instance, you must declare a public static method. 
  • Now, you need to execute “lazy initialization” inside the static method. This should essentially develop a new object on its first call and place that into the static field. The static method must always return that particular instance on all subsequent calls.

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  • Make the constructor private. The class’s static method will still be able to call the constructor, but not the other objects.
  • Please go over the client code and replace all direct calls to Singleton’s constructor with calls to its static creation method.

In Conclusion

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Java is a highly diverse and influential programming language, and it has emerged as a foundation for most of the web and desktop applications in use today. It provides several features and strategies that programmers can use to develop better-performing software in an optimized fashion. Design Patterns are one such strategy adopted by Java programmers to make their codes readable, reusable, and understandable. 

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Design Patterns form one of the basic concepts that any aspiring software developer should look into. The world of software development is broad, and it covers all the aspects from front-end, back-end, database, server, and more. In such a scenario, the need of the hour is to be organized in your approach to learning. At upGrad, you can check out Software Development Courses that will help you build a long-lasting career in the field of software development. Offered in collaboration with IIIT-B, this course is taught by industry experts and gives you a chance to advance your career in the direction of your choice. Check out the course details and register today – the course starts 30th September 2021!


Pavan Vadapalli

Blog Author
Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.

Frequently Asked Questions (FAQs)

1Is Singleton Design Pattern the only design pattern available with Java?

No, Java offers many Design Patterns for programmers, and Singleton is just one of them.

2 What are the types of Design Patterns available with Java?

Java provides broadly three types of design patterns for developers;
1. Creational Design Patterns
2. Structural Design Patterns
3. Behavioral Design Patterns

3 What are the benefits of using Design Patterns in Java?

Using Design Patterns in your code offers the following sure shot benefits:
1. Enhanced readability
2. Improve modularity
3. Easy accessibility
4. Ease in designing and creating programs.

4What are design patterns in Java?

There are some problems that arise frequently and must be resolved in the same manner each time. That well-proven, task-specific approach is called a design pattern. For example, the Builder design pattern is useful to create complex objects from multiple simple objects. It also has the added benefits of construction or representation separation and better construction control. Design patterns are advantageous because they are highly reusable due to their fixed nature. They also make the application design more transparent and subsequently more intuitive. Essentially, they save time by allowing programmers to use code previously written by other developers instead of re-inventing the wheel each time.

5How do design patterns help to minimize errors and code better?

Design patterns help you code better because they lend a certain clarity to the system architecture. They are ideas, not code snippets, which means they are logic-based and not syntax-based. Thus, they give you a generalised solution to a problem that you can adapt to any programming language of your choice. They also help by reducing large, complex objects into simple code segments that are easy to implement, test, and modify. This minimises the chances of error. You can also use design patterns to make the code modular, to an extent, which means that you can add new features to the application without breaking the existing code at any point.

6What are the defining characteristics of Singleton design pattern?

The Singleton design pattern falls under the category of creational patterns. It is used to define a class with only one instance (object). However, this object is globally accessible, though it cannot be modified. To create a singleton class, we require a static member of a class, a private constructor, and a static factory method. The static member creates the actual instance of the class. The private constructor prevents the modification of the instance by actors outside the class. Finally, the static factory method makes the class globally accessible and returns the object to the caller.