Design patterns are general and reusable solutions to a common problem in software design. A pattern design is a description of the solution or a template that can be applied to solve the problem, not a piece of code that can be applied directly. In general, object-oriented patterns show relationships and interactions between classes or objects without specifying the final shape of the classes or objects involved.
Key Points
- Design patterns are reusable solutions to common problems in software design, focusing on relationships and interactions between classes or objects.
- The Singleton Pattern restricts a class to a single instance, useful for shared resources like logger and configuration objects.
- The Observer Pattern establishes a dependency relationship where changes in one object automatically notify and update dependent objects.
Singleton Design Pattern – When and Why to Use a Single Instance
The Singleton design pattern is used to restrict the number of instances of a class to a single object, so it is a way to use a single instance of an object in the application. It provides a global access point to that instance, ensuring that only one object of a given type exists throughout the application lifecycle. The Singleton Pattern is useful in the following cases:
Singleton Pattern – a supporting element for other patterns
Together with Abstract Factory, Builder, Prototype, etc. For example, in the application we want a single factory object to create objects of a certain type. Instead of global variables, a singleton is preferred to global variables because, among other things, it does not pollute the global namespace with unnecessary variables.
Singleton for Shared Objects Across an Application
Singleton is often used in situations where we have objects to access from multiple places in the application:
- logger objects,
- objects that are shared resources (connections, sockets, etc.),
- configuration objects for the application,
- for Factory Objects
Applying the Singleton pattern consists of implementing a method that allows creating a new instance of the class if it does not exist, and returning a reference to it if it already exists. In Java, in order to ensure a single instantiation of the class, the builder must be private, and the instance must be provided by a static, public method.
Factory Pattern Explained: Decoupling Object Creation
Factory patterns are used for objects that generate instances of related classes (implement the same interface, inherit the same abstract class). These are used when we want to isolate the object that needs a court of a certain type, to actually create it.
Additionally, the class that uses the instance does not need to specify exactly the subclass of the object to be created, so you do not have to know all the implementations of that type, but just what features the object has to create. For this reason, Factory is part of the Creational Patterns category because it provides a solution for creating objects.Typical use cases include:
- In libraries / APIs, the user is separate from the actual type of implementation and must use factory methods to obtain certain objects.
- When creating objects is more complex (several stages must be done, etc.), it is more useful to separate the logic needed to instantiate the class sub-type that needs that instance.
By centralizing object creation logic in a factory, the application becomes easier to extend and maintain. New implementations can be introduced with minimal changes to existing code, reducing coupling and improving overall code flexibility. This makes the Factory pattern especially valuable in larger systems where scalability and maintainability are key concerns.
Observer Pattern and Event-Driven Communication Between Objects
The Observer design pattern defines a one-to-many dependency between objects, where a change in the state of one object automatically triggers notifications to all dependent objects. Using this pattern involves the existence of a topic object that has a list of dependent objects as an observer, which it automatically calls every time an action takes place.
Observer belongs to the Behavioral Patterns category, as it focuses on organizing communication between objects based on their behavior and responsibilities rather than their structure.
The observer is used if several classes (observers) depend on the behavior of another class (subject) in situations like:
- when a core class implements the main business logic and other classes monitor or react to its results,
- when actions performed by one class need to be represented or handled in different ways by other classes.
In all these cases, observer objects react to changes in the subject through notifications initiated by the subject’s methods, ensuring loose coupling and improved flexibility in event-driven systems.
Summary: Singleton, Factory, and Observer Patterns
Choosing the right pattern depends on the specific problem being solved, the system architecture, and the desired level of flexibility. For developers working across different areas of system design, understanding how patterns apply beyond pure application logic can also be valuable. You may find it useful to explore related technical topics, such as the location of the wallpapers used in Windows 10 desktop and lock screen, which demonstrates how structured approaches and system knowledge play a role in practical problem solving. Applying design patterns thoughtfully can significantly improve code quality and long-term project sustainability.
