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Svyatoslav Yefimov

Master Object Oriented Design Patterns in Java: Best Practices and Tips



Object Oriented Design Patterns in Java PDF Free




If you are a Java developer who wants to improve your skills and learn how to write better code, you might be interested in learning about object oriented design patterns. Design patterns are reusable solutions to common problems that arise in software development. They can help you write code that is more readable, maintainable, testable and extensible. In this article, we will explain what design patterns are, what types of design patterns exist, how to use some examples of design patterns in Java, and how to learn more about them. We will also provide you with a link to download a free PDF book that covers object oriented design patterns in Java.




Object Oriented Design Patterns In Java Pdf Free



What are design patterns?




Design patterns are general and reusable solutions to common problems that occur in software design. They are not specific code snippets or algorithms, but rather templates or guidelines that can be applied to different situations. Design patterns capture the best practices and experience of software developers who have faced similar challenges before. They can help you avoid reinventing the wheel and make your code more understandable and adaptable.


Some of the benefits of using design patterns are:


  • They can improve the quality and performance of your code by reducing complexity, duplication and errors.



  • They can make your code easier to read and understand by following consistent naming conventions and structures.



  • They can make your code more flexible and extensible by allowing you to change or add functionality without modifying existing code.



  • They can make your code more testable and maintainable by separating concerns and responsibilities.



  • They can facilitate communication and collaboration among developers by providing a common vocabulary and reference.



Types of design patterns




Design patterns can be classified into three main categories based on their purpose and scope:


  • Creational patterns: These patterns deal with the creation and initialization of objects. They abstract the details of object creation and provide different ways to instantiate objects depending on the context and requirements. Some examples of creational patterns are singleton, factory, builder, prototype and abstract factory.



  • Structural patterns: These patterns deal with the composition and arrangement of objects. They define how objects interact and relate to each other to form larger structures. They can help you simplify complex systems by reducing coupling and increasing cohesion. Some examples of structural patterns are adapter, bridge, composite, decorator, facade, flyweight and proxy.



  • Behavioral patterns: These patterns deal with the behavior and communication of objects. They define how objects collaborate and coordinate to achieve a common goal. They can help you encapsulate algorithms, implement state machines, handle events and requests, and support undo/redo operations. Some examples of behavioral patterns are observer, strategy, command, iterator, mediator, memento, state, template method, chain of responsibility and visitor.



Examples of design patterns in Java




In this section, we will briefly describe how to implement some common design patterns in Java using simple examples. Note that these are not the only ways to implement these patterns, and that there may be variations depending on the context and requirements.


Singleton




The singleton pattern ensures that only one instance of a class exists in the application. It also provides a global access point to that instance. The singleton pattern can be useful when you need to control access to a shared resource or coordinate actions across the system.


To implement the singleton pattern in Java, you need to:


  • Declare a private static variable that holds the single instance of the class.



  • Declare a private constructor that prevents instantiation from other classes.



  • Declare a public static method that returns the single instance of the class, creating it if necessary.



Here is an example of a singleton class in Java:


```java public class Singleton // A private static variable that holds the single instance of the class private static Singleton instance; // A private constructor that prevents instantiation from other classes private Singleton() // Some initialization code // A public static method that returns the single instance of the class, creating it if necessary public static Singleton getInstance() // If the instance is null, create a new one if (instance == null) instance = new Singleton(); // Return the instance return instance; // Some other methods and fields ``` Factory




The factory pattern defines an interface or an abstract class for creating objects, but lets subclasses or concrete classes decide which class to instantiate. The factory pattern can be useful when you need to create objects without specifying their exact classes, or when you want to delegate the object creation logic to another class.


To implement the factory pattern in Java, you need to:


  • Declare an interface or an abstract class that defines the common behavior of the objects that will be created by the factory.



  • Implement the interface or extend the abstract class by different subclasses or concrete classes that represent the specific types of objects that will be created by the factory.



  • Declare a factory class that has a method for creating objects of the interface or abstract class type, and returns them as such. The factory class can use parameters, conditions, configuration files, or other mechanisms to decide which subclass or concrete class to instantiate.



Here is an example of a factory pattern in Java:


```java // An interface that defines the common behavior of the objects that will be created by the factory public interface Shape // A method to draw the shape void draw(); // A subclass that implements the interface and represents a specific type of object public class Circle implements Shape // A method to draw the shape @Override public void draw() System.out.println("Drawing a circle"); // Another subclass that implements the interface and represents another specific type of object public class Square implements Shape // A method to draw the shape @Override public void draw() System.out.println("Drawing a square"); // A factory class that has a method for creating objects of the interface type, and returns them as such public class ShapeFactory // A method to create objects of the interface type, using a parameter to decide which subclass to instantiate public Shape getShape(String shapeType) // If the parameter is circle, return a new circle object if (shapeType.equalsIgnoreCase("circle")) return new Circle(); // If the parameter is square, return a new square object else if (shapeType.equalsIgnoreCase("square")) return new Square(); // If the parameter is neither, return null else return null; ``` Adapter




The adapter pattern allows two incompatible interfaces to work together by converting one interface into another. The adapter pattern can be useful when you want to use an existing class that does not match the expected interface, or when you want to create a reusable class that can cooperate with different classes with different interfaces.


To implement the adapter pattern in Java, you need to:


  • Declare an interface or an abstract class that defines the target interface that you want to use.



  • Implement the interface or extend the abstract class by a concrete class that represents the adapter. The adapter class should have a reference to an object of the adaptee interface or class type, and delegate or translate the requests from the target interface to the adaptee interface or class.



  • Declare an interface or a class that defines the adaptee interface or class that you want to adapt.



  • Implement the interface or extend the class by a concrete class that represents the adaptee. The adaptee class should provide the functionality that needs to be adapted.



Here is an example of an adapter pattern in Java:


```java // An interface that defines the target interface that you want to use public interface MediaPlayer // A method to play an audio file of a given format and name void play(String audioType, String fileName); // A concrete class that implements the interface and represents the adapter Observer




The observer pattern defines a one-to-many relationship between a subject and multiple observers. The subject maintains a list of observers and notifies them of any state changes. The observers can register and unregister themselves from the subject, and update their behavior according to the state of the subject. The observer pattern can be useful when you want to implement a publish-subscribe mechanism, or when you want to decouple the objects that depend on each other.


To implement the observer pattern in Java, you need to:


  • Declare an interface or an abstract class that defines the subject. The subject should have methods to attach, detach and notify observers, and to get and set its state.



  • Implement the interface or extend the abstract class by a concrete class that represents the subject. The subject should have a list of observers and implement the methods to manage them and notify them of state changes.



  • Declare an interface or an abstract class that defines the observer. The observer should have a method to update its behavior according to the state of the subject.



  • Implement the interface or extend the abstract class by different concrete classes that represent the observers. The observers should register themselves with the subject and implement the update method accordingly.



Here is an example of an observer pattern in Java:


```java // An interface that defines the subject public interface Subject // A method to attach an observer to the subject void attach(Observer observer); // A method to detach an observer from the subject void detach(Observer observer); // A method to notify all observers of a state change void notifyObservers(); // A method to get the state of the subject int getState(); // A method to set the state of the subject void setState(int state); // A concrete class that implements the interface and represents the subject public class ConcreteSubject implements Subject // A list of observers private List observers; // A state variable private int state; // A constructor that initializes the list of observers and the state variable public ConcreteSubject() observers = new ArrayList(); state = 0; // A method to attach an observer to the subject @Override public void attach(Observer observer) // Add the observer to the list observers.add(observer); // A method to detach an observer from the subject @Override public void detach(Observer observer) // Remove the observer from the list observers.remove(observer); // A method to notify all observers of a state change @Override public void notifyObservers() // Loop through all observers and call their update method with the current state for (Observer observer : observers) observer.update(state); // A method to get the state of the subject @Override public int getState() // Return the state variable return state; // A method to set the state of the subject @Override public void setState(int state) // Set the state variable this.state = state; // Notify all observers of a state change notifyObservers(); // An interface that defines the observer public interface Observer // A method to update its behavior according to the state of the subject void update(int state); // A concrete class that implements the interface and represents an observer public class ConcreteObserver implements Observer // A reference to the subject private Subject subject; // A name for identification private String name; // A constructor that takes a subject and a name as parameters and registers itself with the subject public ConcreteObserver(Subject subject, String name) this.subject = subject; this.name = name; subject.attach(this); // A method to update its behavior according to the state of the subject @Override public void update(int state) // Print a message with its name and the current state of the subject System.out.println(name + " received an update from subject: " + state); // Do some other actions based on the state of the subject // ... ``` Strategy




The strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. The strategy pattern can be useful when you want to vary the behavior of an object at runtime, or when you want to avoid hard-coding the logic of an algorithm