Design patterns are reusable solutions to common programming problems which provide solutions in a standardize way to common software development problems. It means Design patterns help you solve common design problems and increase code reusability by providing tested and tried approaches.
A well-known example of the design pattern is the Singleton pattern. The Singleton pattern is used when you need only a single instance of a class, especially when the class takes up too many resources, such as database connections. It ensures that there is only one instance of a class throughout the application and provides a global point of access to that instance. In PHP, the implementation of Singleton design pattern looks like the below example:
class Singleton
{
// Hold the class instance.
private static $instance = null;
private function __construct()
{
// The expensive process (e.g.,db connection) goes here
}
public static function getInstance()
{
if (self::$instance == null)
{
self::$instance = new Singleton();
}
return self::$instance;
}
public function avoidClone()
{
trigger_error(‘Clone is not allowed ‘, E_USER_ERROR);
}
public function avoidWakeUpSerializing()
{
throw new \Exception(‘Wake up and Serializing is not allowed ‘);
}
}
//how to use singleton class
$instance1 = Singleton::getInstance();
$instance2 = Singleton::getInstance();
$instance3 = Singleton::getInstance();
var_dump($instance1 === $instance2); // true
var_dump($instance1 === $instance3); // true
$instance4 = clone $instance1; // Cloning forbidden
$instance5 = unserialize(serialize($instance1)); //Unserialization forbidden
$instance6 = new Singleton(); //Constructor is private, can’t be instantiated from outside.
In this example, the Singleton design pattern is used to limit the number of instances that can be created from a class to one. This ensures that there is only one instance of the class in the application, which can be accessed by other classes that need it.
Design patterns are reusable solutions to common programming challenges.
They provide a way to communicate common design problems and solutions between developers.
There are three types of design patterns: creational, structural, and behavioral.
Creational patterns deal with object creation mechanisms, trying to create objects in a suitable way for the situation.
Structural patterns are concerned with object composition and provide ways to create relationships between objects.
Behavioral patterns are concerned with object interaction and communication and provide ways to define a system of interactions.
Design patterns are language-independent and platform-independent.
They do not provide solutions to all problems, and there can be multiple types of patterns that solve the same problem.
They promote software development best practices, such as abstraction, encapsulation, inheritance, and polymorphism.
Design patterns can improve code readability, maintainability, and scalability.
What is the purpose of the Singleton pattern, and how does it work?
Answer: The Singleton pattern is used to ensure that a class has only one instance, and provide a global point of access to it. It works by creating a private constructor and a static instance variable, and ensuring that only one instance is created by checking if the variable is null before returning it.
How do you implement the Factory method pattern in Java?
Answer: To implement the Factory method pattern in Java, you can create an interface or abstract class that defines a method for creating objects of a certain type. Then, you can create concrete classes that implement this interface or extend the abstract class, and provide their own implementations of the creation method.
What is the Observer pattern, and how is it different from the Publisher-Subscriber pattern?
Answer: The Observer pattern is used to notify multiple objects of changes to a single subject object. The Publisher-Subscriber pattern is similar, but allows subscribers to only receive updates for specific events. The key difference is that in the Observer pattern, observers register directly with the subject, whereas in the Publisher-Subscriber pattern, they register with a central event manager.
When would you use the Decorator pattern, and how do you implement it in Java?
Answer: The Decorator pattern is used to add new functionality to an existing object, without changing its class. This can be useful when you want to add features to a class that you can’t edit, or when you want to add features dynamically at runtime. To implement the Decorator pattern in Java, you can create a Wrapper class that extends the original class, and adds new functionality in its methods.
What is the Template method pattern, and how does it work?
Answer: The Template method pattern is used to define the structure of an algorithm, but allow subclasses to provide their own implementations of certain steps. This can be useful when you want to enforce a certain order of operations, but allow flexibility in how each step is performed. It works by defining a skeleton method in the base class, which calls abstract methods that must be implemented by subclasses.