nagswarnaa/DesignPatterns

Before diving into the design patterns, it's essential to understand the SOLID principles, which lay the foundational knowledge necessary for understanding design patterns.

SOLID Principles:

Single Responsibility Principle
Open/Closed Principle
Liskov Substitution Principle
Interface Segregation Principle
Dependency Inversion Principle

Let's dive deep into each one these along with examples:

1. Single Responsibility Principle

A class should have only one reason to change, means a class should do only one job or should have only one responsibility.

Let's dive deeper into the Single Responsibility Principle (SRP) with a practical example. Consider a system for managing a library's book inventory. A common violation of SRP occurs when a class takes on responsibilities that should be separated into different classes.

class LibraryBook:
  def __init__(self, title, author, isbn):
      self.title = title
      self.author = author
      self.isbn = isbn
      self.location = None

  def check_in(self, location):
      self.location = location
      # Logic to add the book back to the library inventory

  def check_out(self):
      self.location = 'Checked out'
      # Logic to remove the book from the library inventory

  def save(self):
      # Logic to save the book details to a database

Above example is a violation of SRP Principle since it has multiple responsibilities, Checking-in the books(Inventory), persisting book data to a database, managing book properties. Refactoring above code adhering to SRP principle:

class LibraryBook:
    def __init__(self, title, author, isbn):
        self.title = title
        self.author = author
        self.isbn = isbn

class InventoryManager:
    def __init__(self):
        self.books = {}  # Stores book ISBNs and their locations

    def check_in(self, isbn, location):
        self.books[isbn] = location
        # Additional logic for adding the book back to inventory

    def check_out(self, isbn):
        self.books[isbn] = 'Checked out'
        # Additional logic for removing the book from inventory

class BookPersistence:
    def save_book(self, library_book):
        # Logic to save the book details to a database
        pass

    def load_book(self, isbn):
        # Logic to load book details from a database
        pass

However, following the Single Responsibility Principle (SRP) often leads to a design with more classes, each handling a single responsibility. Applying SRP (and other design principles) requires balance. Over-segmentation can lead to an excessive number of classes, making the system overly complex and potentially difficult to navigate. The key is to identify genuinely distinct responsibilities that justify separation into different classes. The goal is to enhance the maintainability, scalability, and understandability of the code without overcomplicating the design.

In the Java Collections Framework, the principle of Single Responsibility is followed, ensuring that each class has one and only one reason to change. This principle is a key aspect of SOLID design principles, promoting a cleaner, more modular structure.

A clear example of the Single Responsibility Principle (SRP) in the Java Collections Framework is the distinction between the List, Set, and Map interfaces, each serving a distinct purpose:

List Interface: Represents an ordered collection of elements that can contain duplicate values. It is responsible for maintaining the insertion order of the elements, and it allows positional access and insertion of elements. Classes like ArrayList and LinkedList implement the List interface, each with a specific approach to data storage and access, but all adhering to the single responsibility of managing an ordered collection.
Set Interface: Represents a collection that cannot contain duplicate elements. It is designed for scenarios where uniqueness of the elements is crucial. Implementations of the Set interface, such as HashSet and TreeSet, ensure that no two elements in the collection are the same, adhering strictly to their single responsibility of managing a set of unique elements.
Map Interface: Represents a collection of key-value pairs, where each key is unique. This interface is responsible for associating unique keys to specific values, allowing for efficient retrieval, update, and deletion of entries based on the key. Classes like HashMap and TreeMap implement the Map interface, each ensuring that they adhere to the single responsibility of managing a collection of key-value pairs where keys are unique.

2. Open/Closed Principle

A Software entity(class, module) should be open for extension and closed for modification. This principle aims to allow systems to grow and change with new requirements without directly modifying existing source code, thereby reducing the risk of introducing new bugs in previously tested and validated code.

Open for Extension: You should be able to extend the behavior of a module if the requirements of the application change, or to add new features.
Closed for Modification: Extending the behavior of a module should not require changing the source code of the module itself. Instead, the new behavior should be added by creating new entities.

How to Apply OCP

Applying OCP typically involves the use of interfaces or abstract classes to abstract away the concrete implementations of behaviors or algorithms. By programming to an interface, your system can easily incorporate new behaviors by adding new classes that implement these interfaces without changing the existing code.

  from abc import ABC, abstractmethod
 class ReportGenerator(ABC):
    @abstractmethod
    def generate_report(self, content):
        pass

class PdfReportGenerator(ReportGenerator):
    def generate_report(self, content):
        # Logic to generate a PDF report
        print("Report generated in PDF format.")

class HtmlReportGenerator(ReportGenerator):
    def generate_report(self, content):
        # Logic to generate an HTML report
        print("Report generated in HTML format.")

The above system is open for extension because you can add new report formats by creating new classes that implement the ReportGenerator interface. It is closed for modification because you do not need to change existing classes to add new types of reports.

The Open/Closed Principle, one of the SOLID design principles, states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This principle promotes the idea of designing your modules so that new functionality can be added without changing the existing code.

In the Java Collections Framework, the Open/Closed Principle is exemplified by the design of collection interfaces and their implementation classes. Let's focus on the Collection interface and its relationship with various concrete classes like ArrayList, LinkedList, HashSet, and TreeSet.

Collection Interface: This is a root interface in the Collections Framework. It declares the essential operations that all collections will have, such as add(), remove(), size(), iterator(), etc. However, it does not provide any direct implementations of these operations.
Concrete Implementations: Concrete classes like ArrayList, LinkedList, HashSet, and TreeSet implement the Collection interface. Each of these classes provides its own implementation of the methods defined in the interface, optimized for different use cases (e.g., ArrayList for fast random access, LinkedList for efficient insertions/deletions, HashSet for unique elements, and TreeSet for sorted unique elements).

The Open/Closed Principle is followed in this design in the following ways:

Closed for Modification: The Collection interface and its methods define a contract that all implementing classes agree to fulfill. This interface is closed for modifications; adding new methods to it would require changes in all implementing classes, which is not desirable.
Open for Extension: Despite being closed for modifications, the Collection framework is open for extension in several ways:
- New classes can implement the Collection interface to create a new type of collection that adheres to the defined contract.
- Existing classes can be extended to add new behavior. For example, you could extend ArrayList to create a NotifyingArrayList that emits events every time the collection is modified.
- The framework can be extended with new interfaces and classes without altering the existing code. This has been seen over time with additions like the Queue interface and its implementations. The Liskov Substitution Principle (LSP) is one of the five SOLID principles of object-oriented design, introduced by Barbara Liskov in 1987. LSP formalizes a foundational concept for creating maintainable, scalable, and robust object-oriented systems. It focuses on ensuring that inheritance hierarchies are designed so that derived classes can be used in place of their base classes without disrupting the correctness of the program.

3. Liskov Substitution Principle

"Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program."

In simpler terms, if class B is a subtype of class A, then we should be able to replace A with B without affecting the behavior of our program. This implies that B should not weaken the preconditions of A and should meet the postconditions of A, ensuring that B can stand in for A without any issues.

LSP is crucial for the following reasons:

Reliability: Ensures that a subclass can stand in for its superclass without causing errors, leading to more reliable software.
Reusability: Promotes the reuse of base classes and interfaces by guaranteeing that subclasses fulfill the contracts defined by their base classes.
Maintainability: Facilitates easier maintenance of the codebase by ensuring that changes in subclasses do not break the expected behavior of the base class.

Applying LSP

To adhere to the Liskov Substitution Principle:

Ensure Behavioral Compatibility: Subclasses should not only inherit the interface of their base class but also its behavior. This means implementing the methods of the base class in a way that doesn’t alter their expected behavior.
Preserve Invariants: Any rules or conditions that are true for the base class should also be true for the subclass.
Avoid Weakening Preconditions: The conditions under which a subclass method can be called should not be more restrictive than those of its base class.
Avoid Strengthening Postconditions: The conditions after a subclass method has been called should not promise more than what the base class method does.
Substitute Throwability: If a method in the base class is not supposed to throw certain exceptions, the subclass method should adhere to the same constraint.

Here's a simple Java code example that illustrates the LSP violation with the Rectangle and Square scenario described:

// Base class
class Rectangle {
    private int width;
    private int height;

    public void setWidth(int width) {
        this.width = width;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }

    public int getArea() {
        return width * height;
    }
}

// Subclass that violates LSP
class Square extends Rectangle {
    @Override
    public void setWidth(int width) {
        super.setWidth(width);
        super.setHeight(width); // Violation: Changing width changes height
    }

    @Override
    public void setHeight(int height) {
        super.setWidth(height); // Violation: Changing height changes width
        super.setHeight(height);
    }
}

public class LSPDemo {
    public static void main(String[] args) {
        Rectangle rect = new Square();
        rect.setWidth(5);
        rect.setHeight(10);

        // Expectation: area == 50, but due to LSP violation, area will be 100
        System.out.println("Expected area of 5x10 rectangle: 50");
        System.out.println("Actual area: " + rect.getArea());
    }
}

In this example:

The Rectangle class has setWidth() and setHeight() methods that independently set the width and height of a rectangle.
The Square class extends Rectangle but violates LSP because overriding the setWidth() and setHeight() methods to ensure the square's sides are equal leads to a situation where changing the width also changes the height and vice versa. This behavior is not expected from the base class perspective.

When a Rectangle reference points to a Square object and attempts to set the width and height to different values, the end result does not match the expectation based on the behavior defined in Rectangle. This demonstrates the LSP violation, where Square cannot be used as a substitute for Rectangle without altering the correctness of the program.

To adhere to LSP, one could avoid such an inheritance structure and instead use interfaces or favor composition over inheritance, ensuring that subclasses can indeed be substituted for their base class without unexpected behavior. In the Java Collections Framework, the Liskov Substitution Principle (LSP) is followed thoroughly to ensure that subclasses of the collection interfaces can be used interchangeably without breaking the functionality. A prominent example of LSP in action within the Java Collections Framework is the relationship between the List interface and its implementing classes, such as ArrayList and LinkedList.

Consider the List interface and its methods, such as add(E e), get(int index), remove(int index), etc. Both ArrayList and LinkedList implement the List interface and adhere to the contracts defined by it. This means you can substitute an ArrayList with a LinkedList in a piece of code without altering the correctness of the program from the perspective of list operations.

Here is a simple code example to illustrate this:

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;

public class ListExample {
    public static void main(String[] args) {
        List<String> arrayList = new ArrayList<>();
        List<String> linkedList = new LinkedList<>();
        
        fillList(arrayList);
        fillList(linkedList);

        System.out.println("ArrayList contents: " + arrayList);
        System.out.println("LinkedList contents: " + linkedList);
    }

    public static void fillList(List<String> list) {
        list.add("Java");
        list.add("Python");
        list.add("C++");
    }
}

In this example:

The fillList method accepts a List<String> argument and adds several strings to it. This method illustrates that you can use an ArrayList or a LinkedList (or any other class that implements the List interface) interchangeably without needing to change the method's implementation. The fillList method doesn't need to know the specific type of List it's working with, thanks to the LSP adherence in the design of these classes.
Both ArrayList and LinkedList provide their own implementation of the List interface methods, optimized for their respective data structures, but from the perspective of a List user, they can be used interchangeably.

This example showcases that the List interface and its implementers (ArrayList, LinkedList, etc.) follow the Liskov Substitution Principle. It allows for flexibility and interoperability in code that uses the Java Collections Framework, enabling developers to choose the specific implementation that best suits their performance characteristics and requirements without altering the correctness of the program.

The "I" in SOLID principles stands for the Interface Segregation Principle (ISP), which states that no client should be forced to depend on methods it does not use. Essentially, ISP suggests that it's better to have many smaller, more specific interfaces rather than a single, do-all interface. This principle helps in minimizing the impact of changes, improving code organization, and supporting high cohesion with low coupling in software designs.

4. Interface Segregation Principle

ISP aims to reduce the side effects and frequency of required changes by splitting a wide set of actions into smaller and more specific ones. The principle encourages designing interfaces that are client-specific rather than general-purpose, so clients will only know about the methods that are of interest to them.

Importance

Minimizes Code Reusability Issues: By adhering to ISP, systems become more flexible and adaptable to change. Clients can implement only the interfaces their need, reducing the dependencies on unused code.
Enhances System Maintainability: Smaller, well-defined interfaces are easier to implement, test, and maintain over time.
Supports High Cohesion: Ensures that modules or components have tightly related and focused functionalities.
Encourages Decoupling: Clients are less likely to be affected by changes in unrelated system parts, leading to systems that are easier to refactor, change, and understand.

Applying ISP

To apply ISP effectively:

Identify the Clients: Understand who or what will be using the interface. A client can be another class or module in the system.
Define Specific Interfaces: Based on the client's needs, define interfaces that contain only the methods required by the client.
Implement Interfaces: Classes should implement these specific interfaces, ensuring that they don't carry additional unnecessary methods that aren't of use to the client.

Comsider Below example

Consider an all-in-one interface for a smart device:

interface SmartDevice {
    void print();
    void fax();
    void scan();
}

This design forces all implementing classes to define methods for print, fax, and scan, even if a specific device does not support all these functions. This violates ISP.

A better approach following ISP would be to segregate the SmartDevice interface into more specific interfaces:

interface Printer {
    void print();
}

interface Fax {
    void fax();
}

interface Scanner {
    void scan();
}

Now, a device that only supports printing need only implement the Printer interface:

class SimplePrinter implements Printer {
    public void print() {
        // Implementation for print
    }
}

And a multifunctional device could implement multiple interfaces:

class MultiFunctionalPrinter implements Printer, Scanner, Fax {
    public void print() {
        // Implementation for print
    }

    public void scan() {
        // Implementation for scan
    }

    public void fax() {
        // Implementation for fax
    }
}

In the Java Collections Framework, the Interface Segregation Principle (ISP) is inherently applied through its design, which favors specific interfaces for specific purposes rather than a single, monolithic interface for all collection types. This design allows for flexibility and ensures that implementing classes only need to provide functionality relevant to their specific type of collection, adhering to the ISP.

Example of ISP in Java Collections Framework

Consider how Java's Collection Framework segregates interfaces for different collection types:

Collection Interface: The root interface in the Collections Framework hierarchy. It represents a group of objects, known as its elements. This interface is at the top level and is implemented by various other collection interfaces like List, Set, and Queue.
List Interface: An ordered collection (also known as a sequence). Lists can contain duplicate elements. The user can access elements by their integer index (position in the list), and search for elements in the list.
Set Interface: A collection that cannot contain duplicate elements. It models the mathematical set abstraction and is used to represent sets, such as the deck of cards.
Queue Interface: A collection used to hold multiple elements prior to processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations. Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner.

Each of these interfaces targets a specific type of collection with specific behaviors and properties, adhering to the Interface Segregation Principle. Clients can use these interfaces without being forced to implement methods that are irrelevant to the type of collection they are interested in.

Applying ISP in Practice

Here's a simplified example of how you might apply ISP when working with these interfaces:

import java.util.ArrayList;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;

public class CollectionExample {
    public static void main(String[] args) {
        // List example
        ArrayList<String> arrayList = new ArrayList<>();
        arrayList.add("Item 1");
        arrayList.add("Item 2");
        System.out.println("ArrayList: " + arrayList);

        // Set example
        HashSet<String> hashSet = new HashSet<>();
        hashSet.add("Item 1");
        hashSet.add("Item 1");
        System.out.println("HashSet (duplicates not allowed): " + hashSet);

        // Queue example
        Queue<String> queue = new LinkedList<>();
        queue.offer("First");
        queue.offer("Second");
        System.out.println("Queue: " + queue);
    }
}

In this example:

ArrayList implements the List interface, providing an ordered, index-accessible collection that allows duplicates.
HashSet implements the Set interface, providing a collection that ensures uniqueness of its elements.
LinkedList implements both the List and Queue interfaces, showing how a class can serve multiple collection roles, thanks to the segregation of interfaces in the Collections Framework.

This approach adheres to ISP by ensuring that a class is not forced to implement interfaces and methods that it does not use.

5. Dependency Inversion Principle

 DIP is a crucial principle in software design aimed at reducing dependencies between high-level modules (which provide complex logic) and low-level modules (which provide utility features or basic operations) by introducing an abstraction layer.

Understanding High-Level and Low-Level Modules

High-Level Modules are classes or components that implement business rules or logic. They operate on a higher level of abstraction, orchestrating the behavior of the application.
Low-Level Modules are the workhorses that perform specific operations, like database access, network communication, or device control. They are detailed implementations that high-level modules rely on to function.

The Problem DIP Addresses

Without DIP, high-level modules directly depend on low-level modules, making the system rigid, difficult to change, and hard to test. For example, if a high-level module OrderProcessor directly creates and uses a low-level module MySQLDatabase for data persistence, it's tightly coupled to that specific implementation. If you decide to switch databases, you'd need to modify the OrderProcessor, violating the open/closed principle.

The DIP Solution

DIP solves this problem by inverting the traditional dependency relationship:

Both Should Depend on Abstractions: Instead of high-level modules depending on low-level modules, both should depend on abstractions. An abstraction is a layer that provides a general interface for interaction, such as interfaces or abstract classes in Java.
Abstractions Should Not Depend on Details: Interfaces or abstract classes should not be tailored to specific implementations but should define a general contract that any implementation can fulfill.

How to Apply DIP

Define Interfaces or Abstract Classes: Identify what operations high-level modules need from low-level modules and define these operations in interfaces or abstract classes. This step creates the abstraction layer.
Implement Interfaces for Low-Level Modules: Create concrete implementations of the interfaces for your low-level modules. These are the specific details that fulfill the contract defined by the abstraction.
Inject Dependencies: High-level modules should be designed to accept any implementation of the interface they depend on. This is typically achieved through dependency injection, where the specific instances of the low-level modules are provided to the high-level modules (e.g., via a constructor, method parameter, or using a dependency injection framework).

Detailed Example

Let's expand on the Button and Lamp example with a focus on dependency injection for better clarity:

// Step 1: Define an interface as an abstraction layer
interface Device {
    void turnOn();
    void turnOff();
}

// Step 2: Implement the interface with a low-level module
class Lamp implements Device {
    @Override
    public void turnOn() {
        System.out.println("Lamp is now on.");
    }

    @Override
    public void turnOff() {
        System.out.println("Lamp is now off.");
    }
}

// Another low-level module
class Fan implements Device {
    @Override
    public void turnOn() {
        System.out.println("Fan is now on.");
    }

    @Override
    public void turnOff() {
        System.out.println("Fan is now off.");
    }
}

// Step 3: High-level module that depends on the abstraction (interface)
class RemoteControl {
    private Device device;

    // Dependency injection via constructor
    public RemoteControl(Device device) {
        this.device = device;
    }

    public void togglePower() {
        if (Math.random() > 0.5) {
            device.turnOn();
        } else {
            device.turnOff();
        }
    }
}

// Demonstrating DIP with dependency injection
public class HomeAutomation {
    public static void main(String[] args) {
        Device lamp = new Lamp();
        Device fan = new Fan();

        RemoteControl controlForLamp = new RemoteControl(lamp);
        RemoteControl controlForFan = new RemoteControl(fan);

        controlForLamp.togglePower();
        controlForFan.togglePower();
    }
}

In this example:

Device is the abstraction that both high-level and low-level modules depend on.
Lamp and Fan are low-level modules that implement the Device interface. They can be replaced or extended without affecting the high-level module (RemoteControl).
RemoteControl is a high-level module that operates on the Device abstraction. It's designed to control any device, demonstrating DIP by depending on an abstraction rather than concrete implementations.

Scenario: Data Processing and Storage

Imagine you have a high-level module responsible for processing data and then storing it. Without applying DIP, this module might directly depend on a specific collection implementation (e.g., ArrayList) for its storage needs. However, this direct dependency makes the module less flexible and more tightly coupled to a specific implementation of the Java Collections Framework.

Step 1: Define an Interface as an Abstraction

First, define an interface that abstracts the concept of data storage. This interface declares methods for adding and retrieving data, without specifying the underlying storage mechanism.

interface DataStorage<T> {
    void add(T item);
    T get(int index);
}

Step 2: Implement the Interface with Specific Collections

Next, create concrete implementations of the DataStorage interface using different Java Collections. These implementations are the low-level modules in this context.

class ListDataStorage<T> implements DataStorage<T> {
    private List<T> list = new ArrayList<>();

    @Override
    public void add(T item) {
        list.add(item);
    }

    @Override
    public T get(int index) {
        return list.get(index);
    }
}

class SetDataStorage<T> implements DataStorage<T> {
    private Set<T> set = new HashSet<>();
    private List<T> list = new ArrayList<>();

    @Override
    public void add(T item) {
        if (set.add(item)) { // Add item to set to ensure uniqueness, and list for retrieval
            list.add(item);
        }
    }

    @Override
    public T get(int index) {
        return list.get(index);
    }
}

Step 3: High-Level Module That Depends on the Abstraction

Create a high-level module, such as a data processor, that operates on the DataStorage abstraction. This module is designed to work with any data storage implementation that conforms to the DataStorage interface.

class DataProcessor<T> {
    private DataStorage<T> storage;

    public DataProcessor(DataStorage<T> storage) {
        this.storage = storage;
    }

    public void processData(T data) {
        // Process data (details omitted for brevity)
        storage.add(data);
    }

    public T retrieveData(int index) {
        return storage.get(index);
    }
}

Demonstrating Dependency Inversion

public class Application {
    public static void main(String[] args) {
        DataStorage<String> listStorage = new ListDataStorage<>();
        DataStorage<String> setStorage = new SetDataStorage<>();

        DataProcessor<String> processorUsingList = new DataProcessor<>(listStorage);
        DataProcessor<String> processorUsingSet = new DataProcessor<>(setStorage);

        processorUsingList.processData("Data1");
        processorUsingList.processData("Data2");

        processorUsingSet.processData("Data1");
        processorUsingSet.processData("Data1"); // Duplicate, will not be added in set

        System.out.println(processorUsingList.retrieveData(0)); // Outputs: Data1
        System.out.println(processorUsingSet.retrieveData(0)); // Outputs: Data1
    }
}

In this example:

DataStorage<T> serves as an abstraction for different ways of storing data, allowing the DataProcessor (a high-level module) to depend on an abstraction rather than concrete implementations.
ListDataStorage<T> and SetDataStorage<T> are concrete implementations of the DataStorage interface, showing how low-level

The image you've uploaded seems to contain a list of Structural Design Patterns. Let's go over each one with a detailed explanation and examples:

Design Patterns:

Introduction to Structural Patterns

Structural Design Patterns are concerned with how classes and objects are composed to form larger structures. They help ensure that when one part of a structure changes, the entire structure does not need to change. The key principle they use is composition—building complex structures by combining simpler parts.

Decorator Pattern

Intent: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example: Imagine you have a Coffee class and you want to add different mix-ins like milk, sugar, or whipped cream without altering the Coffee class:

// Component
public interface Coffee {
    String getDescription();
    double cost();
}

// Concrete Component
public class SimpleCoffee implements Coffee {
    @Override
    public String getDescription() {
        return "Simple Coffee";
    }

    @Override
    public double cost() {
        return 1.0;
    }
}

// Decorator
public abstract class CoffeeDecorator implements Coffee {
    protected Coffee decoratedCoffee;
    
    public CoffeeDecorator(Coffee coffee) {
        this.decoratedCoffee = coffee;
    }
    
    public String getDescription() {
        return decoratedCoffee.getDescription();
    }
    
    public double cost() {
        return decoratedCoffee.cost();
    }
}

// Concrete Decorator
public class MilkDecorator extends CoffeeDecorator {
    public MilkDecorator(Coffee coffee) {
        super(coffee);
    }
    
    @Override
    public String getDescription() {
        return super.getDescription() + ", Milk";
    }
    
    @Override
    public double cost() {
        return super.cost() + 0.5;
    }
}

Facade Pattern

Intent: Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

Example: Let's say you have a home theater system with a TV, DVD player, and sound system. Instead of controlling each separately, you could have a HomeTheaterFacade that simplifies the process:

public class HomeTheaterFacade {
    private TV tv;
    private DVDPlayer dvdPlayer;
    private SoundSystem soundSystem;

    public HomeTheaterFacade(TV tv, DVDPlayer dvdPlayer, SoundSystem soundSystem) {
        this.tv = tv;
        this.dvdPlayer = dvdPlayer;
        this.soundSystem = soundSystem;
    }

    public void watchMovie(String movie) {
        tv.on();
        soundSystem.on();
        dvdPlayer.on();
        dvdPlayer.play(movie);
    }
    
    public void endMovie() {
        tv.off();
        soundSystem.off();
        dvdPlayer.eject();
        dvdPlayer.off();
    }
}

Adapter Pattern

Intent: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

Example: If you have a USPowerSocket and a European device with a EuropeanPlug, you can't connect the device directly. You need an adapter:

// Target
public interface USPowerSocket {
    void plugIn(USPlugConnector plug);
}

// Adaptee
public interface EuropeanPlugConnector {
    void giveElectricity();
}

// Adapter
public class USPlugAdapter implements USPowerSocket {
    private EuropeanPlugConnector plug;

    public USPlugAdapter(EuropeanPlugConnector plug) {
        this.plug = plug;
    }

    @Override
    public void plugIn(USPlugConnector plug) {
        plug.giveElectricity();
    }
}

Bridge Pattern

Intent: Decouple an abstraction from its implementation so that the two can vary independently.

Example: If you have a RemoteControl abstraction and you want it to work with different devices like TVs and radios without binding it to a specific device's implementation:

// Implementor
public interface Device {
    void turnOn();
    void turnOff();
    void setChannel(int channel);
}

// Concrete Implementor
public class TV implements Device {
    public void turnOn() { /* ... */ }
    public void turnOff() { /* ... */ }
    public void setChannel(int channel) { /* ... */ }
}

// Abstraction
public abstract class RemoteControl {
    protected Device device;
    
    public RemoteControl(Device device) {
        this.device = device;
    }
    
    public abstract void togglePower();
}

// Refined Abstraction
public class BasicRemoteControl extends RemoteControl {
    public BasicRemoteControl(Device device) {
        super(device);
    }
    
    public void togglePower() {
        /* Implementation code using device methods */
    }
}

Composite Pattern

Intent: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects

uniformly.

Example: Imagine you're building a file system where both individual File and Directory should be treated uniformly:

// Component
public interface FileSystemNode {
    void print(String structure);
}

// Leaf
public class File implements FileSystemNode {
    private String name;

    public File(String name) {
        this.name = name;
    }
    
    @Override
    public void print(String structure) {
        System.out.println(structure + name);
    }
}

// Composite
public class Directory implements FileSystemNode {
    private List<FileSystemNode> children = new ArrayList<>();
    private String name;

    public Directory(String name) {
        this.name = name;
    }
    
    public void add(FileSystemNode node) {
        children.add(node);
    }

    @Override
    public void print(String structure) {
        System.out.println(structure + name);
        for (FileSystemNode child : children) {
            child.print(structure + "  ");
        }
    }
}

Flyweight Pattern

Intent: Use sharing to support large numbers of fine-grained objects efficiently.

Example: If you're rendering a forest with millions of trees, you don't want to store color and texture data for each tree individually:

// Flyweight
public class TreeModel {
    private Mesh mesh;
    private Texture bark;
    private Texture leaves;
    
    // Constructor and methods
}

// Context
public class Tree {
    private TreeModel model;
    private double height;
    private double positionX;
    private double positionY;
    
    // Constructor and methods that use model
}

// Flyweight Factory
public class TreeFactory {
    private static Map<String, TreeModel> treeModels = new HashMap<>();

    public static TreeModel getTreeModel(String species) {
        if(!treeModels.containsKey(species)) {
            treeModels.put(species, new TreeModel(species));
        }
        return treeModels.get(species);
    }
}

Proxy Pattern

Intent: Provide a surrogate or placeholder for another object to control access to it.

Example: You want to lazy-load an image in a graphic editor:

// Subject
public interface Image {
    void display();
}

// Real Subject
public class HighResolutionImage implements Image {
    public HighResolutionImage(String imageFilePath) {
        // load image from disk into memory
        // this is a heavy operation
    }
    
    @Override
    public void display() {
        // display the image
    }
}

// Proxy
public class ImageProxy implements Image {
    private HighResolutionImage highResImage;
    private String imageFilePath;

    public ImageProxy(String imageFilePath) {
        this.imageFilePath = imageFilePath;
    }
    
    @Override
    public void display() {
        if(highResImage == null) {
            highResImage = new HighResolutionImage(imageFilePath);
        }
        highResImage.display();
    }
}

Let's start with the creational design patterns

1. Factory Design Pattern

The Factory Design Pattern is one of the most commonly used design patterns in object-oriented programming. It belongs to the creational pattern category, which deals with object creation mechanisms. The factory pattern is particularly useful when a system needs to manage, create, and manipulate a large group of similar objects without necessarily knowing the specifics of each object.

Overview

Purpose: Simplifies object creation in a system by hiding the details of how the objects are created.
Category: Creational design pattern.
Use Case: When you want to create objects of a class and its subclasses, but you want to encapsulate the instantiation logic from the client to enhance modularity and flexibility.

Concept

Factory Method: At its core, the Factory Design Pattern revolves around a single method (often called a factory method) responsible for creating objects. This method usually returns an object of a common interface type, allowing for flexibility in the types of objects that can be created.
Encapsulation: The pattern encapsulates the creation of objects. This means that the details of object creation are not exposed to the client, making the code more modular, easier to maintain, and extend.
Subclasses: It allows for the creation of objects without specifying the exact class of object that will be created. Instead, subclasses of a common superclass are instantiated, which is decided at runtime based on the parameters provided to the factory method.

Components

Product: This is the interface or abstract class defining the objects the factory method needs to create.
Concrete Product: These are the specific implementations of the Product interface.
Creator: An interface that declares the factory method, which returns an object of type Product.
Concrete Creator: Implements or extends the Creator class. The factory method in this class overrides the factory method in the Creator to return a specific Concrete Product.

Benefits

Flexibility and Reusability: The factory pattern allows for the objects to be instantiated at runtime, making the system more flexible and reusable.
Decoupling: The client is decoupled from the creation process of the object, leading to easier maintenance and scalability.
Single Responsibility Principle: The pattern helps in keeping the object creation process in a single place, thus adhering to the Single Responsibility Principle.

Examples in Real Life

Database Connections: A factory can be used to create connections to different types of databases (MySQL, PostgreSQL, etc.) based on configuration.
User Interface Components: A UI framework might use a factory pattern to create different types of UI elements (buttons, text fields, etc.) without exposing the instantiation logic to the client.

Implementation Steps

Define a Product Interface: This interface represents the objects the factory will create.
Create Concrete Products: These are the specific classes that implement the Product interface.
Create a Creator Interface or Class: This includes a method for creating objects of the Product type.
Implement Concrete Creators: These classes override the factory method to create and return instances of Concrete Products.

By employing the Factory Design Pattern, developers can write cleaner, more maintainable, and more scalable code by abstracting the complex creation logic away from the client and by promoting a more modular approach to object creation.

Before diving into the design patterns, it's essential to understand the SOLID principles, which lay the foundational knowledge necessary for understanding design patterns.

SOLID Principles:

1. Single Responsibility Principle

2. Open/Closed Principle

3. Liskov Substitution Principle

Applying LSP

4. Interface Segregation Principle

Importance

Applying ISP

Comsider Below example

Example of ISP in Java Collections Framework

Applying ISP in Practice

5. Dependency Inversion Principle

Understanding High-Level and Low-Level Modules

The Problem DIP Addresses

The DIP Solution

How to Apply DIP

Detailed Example

Scenario: Data Processing and Storage

Step 1: Define an Interface as an Abstraction

Step 2: Implement the Interface with Specific Collections

Step 3: High-Level Module That Depends on the Abstraction

Demonstrating Dependency Inversion

Design Patterns:

Introduction to Structural Patterns

Decorator Pattern

Facade Pattern

Adapter Pattern

Bridge Pattern

Composite Pattern

Flyweight Pattern

Proxy Pattern

1. Factory Design Pattern

Overview

Concept

Components

Benefits

Examples in Real Life

Implementation Steps

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