Design patterns are like the blueprints provided by a seasoned carpenter to a novice, illustrating the best and most efficient ways to solve common problems in software engineering. They are more specific solutions compared to principles like SOLID and offer generalized solutions to tackle recurring design problems effectively.
1. Singleton Pattern (Creational)
Intent:
Ensure a class has only one instance and provides a global point of access to it.
Detailed Explanation:
The Singleton Pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the Java Virtual Machine. It’s particularly useful for providing a global point of access to the object.
Example:
final class Manager {
private static Manager manager;
private Manager() {}
public static Manager getInstance() {
if(manager == null)
manager = new Manager();
return manager;
}
}
In-depth Analysis:
In this example, the Manager class is marked as final to prevent subclassing. The private constructor ensures that the class cannot be instantiated from outside the class. The getInstance() method ensures that the class has only one instance and provides a global point of access to it.
2. Factory Method Pattern (Creational)
Intent:
Define an interface for creating an object but let the subclasses decide which class to instantiate.
Detailed Explanation:
The Factory Method Pattern is used when a class can’t anticipate the class of objects it needs to create. It relies on inheritance and is object-based.
Example:
abstract class GraphicsApplication {
public abstract Shape factoryMethod();
}
class CirclesApplication extends GraphicsApplication {
@Override
public Shape factoryMethod() {
return new Circle();
}
}
In-depth Analysis:
Here, GraphicsApplication is an abstract class with an abstract factoryMethod(). Subclasses like CirclesApplication override this method to instantiate and return the required object, allowing for the creation of objects without specifying the exact class of object that will be created.
3. Facade Pattern (Structural)
Intent:
Provide a unified interface to a set of interfaces in a subsystem to make the subsystem easier to use.
Detailed Explanation:
The Facade Pattern simplifies the interface to a complex subsystem by providing a unified, higher-level interface, making the subsystem easier to use.
Example:
class Facade {
Screen s;
Light l;
Projector p;
public void watchMovie() {
s.down();
l.dim();
p.on();
}
}
In-depth Analysis:
The Facade class in this example simplifies the interaction with subsystem classes (Screen, Light, and Projector), providing a single watchMovie() method to perform all the necessary actions to watch a movie, thus hiding the complexities of the subsystem.
4. Adapter Pattern (Structural)
Intent:
Convert the interface of a class into another interface clients expect.
Detailed Explanation:
The Adapter Pattern allows the interface of an existing class to be used as another interface, enabling interoperability between classes with incompatible interfaces.
Example:
class Adapter implements Target {
Adaptee adaptee;
@Override
public int add(int a, int b) {
return adaptee.sum(a, b);
}
}
In-depth Analysis:
The Adapter class in this example implements the Target interface and contains a reference to an object of Adaptee class, allowing clients to use the Target interface to interact with objects of Adaptee class.
5. Observer Pattern (Behavioral)
Intent:
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Detailed Explanation:
The Observer Pattern is used when a subject needs to notify its observers about a change in its state. It defines a dependency between objects so that when one object changes state, all its dependents are notified.
Example:
class Subject {
public void setState(int value) {
state = value;
notifyAllObservers();
}
}
In-depth Analysis:
In this example, the Subject class has a method setState(int value) that changes the state of the subject and notifies all registered observers about this change, allowing them to update themselves accordingly.
6. Strategy Pattern (Behavioral)
Intent:
Define a family of algorithms, encapsulate each one, and make them interchangeable.
Detailed Explanation:
The Strategy Pattern is used when we want to define a family of algorithms and make them interchangeable. It allows the client to choose the appropriate algorithm at runtime.
Example:
class Sorter {
private SortingStrategy sortingStrategy;
public void performSort(List<Integer> list) {
sortingStrategy.sort(list);
}
}
In-depth Analysis:
In this example, the Sorter class has a reference to the SortingStrategy interface. It has a performSort(List<Integer> list) method that calls the sort(list) method on the SortingStrategy object, allowing the client to use different sorting algorithms interchangeably.
Conclusion:
Design patterns provide generalized solutions to common problems encountered in software design. They encapsulate the experience and insights of seasoned developers, allowing for the development of robust, scalable, and maintainable software. By understanding and applying design patterns effectively, developers can avoid common pitfalls and improve the overall quality of their software.