Design patterns in Java
Design patterns are reusable solutions to common problems that occur in software design. They provide a structured approach to designing software and help improve code organization, maintainability, and flexibility. Here are some commonly used design patterns in Java:
1. Singleton Pattern:
- Ensures that a class has only one instance and provides a global access point to it.
- Example: `java.lang.Runtime`
2. Factory Pattern:
- Provides an interface for creating objects, but lets subclasses decide which class to instantiate.
- Example: `java.util.Calendar`
3. Observer Pattern:
- Defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated automatically.
- Example: `java.util.Observable` and `java.util.Observer`
4. Builder Pattern:
- Separates the construction of complex objects from their representation, allowing the same construction process to create various representations.
- Example: `java.lang.StringBuilder`
5. Decorator Pattern:
- Allows behavior to be added to an individual object dynamically without affecting the behavior of other objects from the same class.
- Example: `java.io.BufferedInputStream`
6. Adapter Pattern:
- Allows the interface of an existing class to be used as another interface that clients expect.
- Example: `java.util.Arrays#asList()`
7. Iterator Pattern:
- Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
- Example: `java.util.Iterator`
8. Proxy Pattern:
- Provides a surrogate or placeholder for another object to control access to it.
- Example: `java.lang.reflect.Proxy`
9. Strategy Pattern:
- Defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.
- Example: `java.util.Comparator`
10. Template Method Pattern:
- Defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Subclasses can redefine certain steps of the algorithm without changing its structure.
- Example: `java.util.AbstractList`
These are just a few examples of design patterns in Java. There are many more design patterns available, each addressing specific design concerns and providing solutions for different scenarios.
Singleton Pattern
The Singleton pattern is a creational design pattern that ensures a class has only one instance and provides a global access point to that instance. It is useful in scenarios where you want to restrict the instantiation of a class to a single object.
Here's an example of implementing the Singleton pattern in Java:
public class Singleton {
private static Singleton instance;
// Private constructor to prevent instantiation from outside the class
private Singleton() {
}
// Public method to provide access to the single instance
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
} // Other methods and properties of the class
// ...
}
In this example, the `Singleton` class has a private constructor to prevent direct instantiation of the class from outside. The class also has a `private static` member variable called `instance`, which holds the single instance of the class.
The `getInstance()` method is used to access the single instance of the class. It first checks if the `instance` variable is `null`, indicating that no instance has been created yet. If so, it creates a new instance using the private constructor. On subsequent calls to `getInstance()`, it simply returns the already created instance.
Here's how you can use the `Singleton` class:
public class Main {
public static void main(String[] args) {
Singleton singleton1 = Singleton.getInstance();
Singleton singleton2 = Singleton.getInstance();
// Both singleton1 and singleton2 refer to the same instance
System.out.println(singleton1 == singleton2); // Output: true
}
}
In the example above, `singleton1` and `singleton2` are two variables that hold the instance of the `Singleton` class. Since the `getInstance()` method always returns the same instance, `singleton1 == singleton2` will evaluate to `true`, indicating that both variables refer to the same object.
The Singleton pattern ensures that there is only one instance of the class throughout the application, and provides a global access point to that instance. This can be useful in scenarios where you need to share a common resource, maintain a single state, or coordinate actions across different parts of the codebase.
Factory Pattern
The Factory pattern is a creational design pattern that provides an interface for creating objects but delegates the actual object creation to subclasses. It allows the client code to create objects without knowing the specific class or implementation details of the created objects. The Factory pattern promotes loose coupling and encapsulation by separating object creation from the client code.
Here's an example of implementing the Factory pattern in Java:
// Product interface
interface Vehicle {
void drive();
}
// Concrete product implementations
class Car implements Vehicle {
@Override
public void drive() {
System.out.println("Driving a car...");
}
}
class Motorcycle implements Vehicle {
@Override
public void drive() {
System.out.println("Riding a motorcycle...");
}
}
// Factory class
class VehicleFactory {
public static Vehicle createVehicle(String type) {
if (type.equalsIgnoreCase("car")) {
return new Car();
} else if (type.equalsIgnoreCase("motorcycle")) {
return new Motorcycle();
}
throw new IllegalArgumentException("Invalid vehicle type: " + type);
}
}
// Client code
public class Main {
public static void main(String[] args) {
Vehicle car = VehicleFactory.createVehicle("car");
car.drive(); // Output: Driving a car...
Vehicle motorcycle = VehicleFactory.createVehicle("motorcycle");
motorcycle.drive(); // Output: Riding a motorcycle...
}
}
In this example, we have a `Vehicle` interface representing the product, with concrete implementations `Car` and `Motorcycle`. The `Vehicle` interface declares the common behavior of all vehicles, which is the `drive()` method.
The `VehicleFactory` class acts as the factory, providing a static method `createVehicle()` that takes a `String` parameter representing the type of vehicle to create. Based on the provided type, the factory method creates and returns the corresponding concrete product instance.
The client code (`Main` class) uses the factory to create different types of vehicles without explicitly knowing the concrete classes. It simply calls the `createVehicle()` method on the factory, passing the desired type as an argument, and then uses the returned `Vehicle` instance to perform operations.
The Factory pattern decouples the client code from the concrete implementation classes, making it easy to introduce new types of vehicles by extending the factory and implementing the corresponding product classes. It centralizes the object creation logic in one place and provides a unified interface for creating objects throughout the application.
Observer Pattern
The Observer pattern is a behavioral design pattern that establishes a one-to-many dependency between objects. It defines a mechanism where multiple observers (also known as subscribers or listeners) are notified and updated automatically when the state of a subject (also known as publisher or observable) changes. The Observer pattern promotes loose coupling between objects by allowing them to communicate without having direct knowledge of each other.
Here's an example of implementing the Observer pattern in Java:
import java.util.ArrayList;
import java.util.List;
// Subject (Observable) interface
interface Subject {
void addObserver(Observer observer);
void removeObserver(Observer observer);
void notifyObservers();
}
// Concrete subject implementation
class NewsAgency implements Subject {
private List<Observer> observers = new ArrayList<>();
private String news;
@Override
public void addObserver(Observer observer) {
observers.add(observer);
}
@Override
public void removeObserver(Observer observer) {
observers.remove(observer);
}
@Override
public void notifyObservers() {
for (Observer observer : observers) {
observer.update(news);
}
}
public void setNews(String news) {
this.news = news;
notifyObservers();
}
}
// Observer interface
interface Observer {
void update(String news);
}
// Concrete observer implementations
class NewsChannel implements Observer {
private String news;
@Override
public void update(String news) {
this.news = news;
displayNews();
}
private void displayNews() {
System.out.println("NewsChannel: " + news);
}
}
class SocialMediaFeed implements Observer {
private String news;
@Override
public void update(String news) {
this.news = news;
displayNews();
}
private void displayNews() {
System.out.println("SocialMediaFeed: " + news);
}
}
// Client code
public class Main {
public static void main(String[] args) {
NewsAgency newsAgency = new NewsAgency();
NewsChannel newsChannel = new NewsChannel();
newsAgency.addObserver(newsChannel);
SocialMediaFeed socialMediaFeed = new SocialMediaFeed();
newsAgency.addObserver(socialMediaFeed);
newsAgency.setNews("Breaking News: Important event happened!");
}
}
In this example, we have a `NewsAgency` class implementing the `Subject` interface. It maintains a list of observers (`Observer` objects) and provides methods to add observers, remove observers, and notify all observers when the news is updated.
The `NewsChannel` and `SocialMediaFeed` classes implement the `Observer` interface. They have an `update()` method that is called by the `NewsAgency` when there is a news update. Each observer updates its internal state with the latest news and performs its specific action, such as displaying the news.
The client code (`Main` class) demonstrates the usage of the Observer pattern. It creates a `NewsAgency` instance and registers `NewsChannel` and `SocialMediaFeed` as observers using the `addObserver()` method. When the `NewsAgency` updates its news by calling `setNews()`, all registered observers are notified automatically and their `update()` methods are invoked.
Running the code will produce the following output:
NewsChannel: Breaking News: Important event happened!
SocialMediaFeed: Breaking News: Important event happened!
As observed, both the `NewsChannel` and `SocialMediaFeed` objects received and displayed the same news update. The Observer pattern allows for flexible and decoupled communication between subjects and observers, enabling multiple objects to be notified and updated in response to changes in a subject's state.
Builder Pattern
The Builder pattern is a creational design pattern that separates the construction of an object from its representation. It allows you to create complex objects step by step and provides a clean and readable way to construct objects with multiple optional parameters. The Builder pattern promotes immutability and provides a flexible and fluent API for object creation.
Here's an example of implementing the Builder pattern in Java:
// Product class
class House {
private String foundation;
private String structure;
private String roof;
private String interior;
private House(Builder builder) {
this.foundation = builder.foundation;
this.structure = builder.structure;
this.roof = builder.roof;
this.interior = builder.interior;
}
// Getters for the house attributes
public String getFoundation() {
return foundation;
}
public String getStructure() {
return structure;
}
public String getRoof() {
return roof;
}
public String getInterior() {
return interior;
}
// Builder class
static class Builder {
private String foundation;
private String structure;
private String roof;
private String interior;
public Builder() {
}
public Builder setFoundation(String foundation) {
this.foundation = foundation;
return this;
}
public Builder setStructure(String structure) {
this.structure = structure;
return this;
}
public Builder setRoof(String roof) {
this.roof = roof;
return this;
}
public Builder setInterior(String interior) {
this.interior = interior;
return this;
}
public House build() {
return new House(this);
}
}
}
// Client code
public class Main {
public static void main(String[] args) {
House.Builder builder = new House.Builder();
builder.setFoundation("Concrete")
.setStructure("Brick")
.setRoof("Shingles")
.setInterior("Modern");
House house = builder.build();
System.out.println("House details:");
System.out.println("Foundation: " + house.getFoundation());
System.out.println("Structure: " + house.getStructure());
System.out.println("Roof: " + house.getRoof());
System.out.println("Interior: " + house.getInterior());
}
}
In this example, we have a `House` class representing the product. It has private attributes for the house's foundation, structure, roof, and interior. The class provides a private constructor that takes a `Builder` object and initializes the attributes.
The `Builder` class is a nested static class within the `House` class. It provides methods to set each attribute of the `House` class and returns the same `Builder` object to support method chaining. The `build()` method creates a new `House` object using the `Builder` object's attributes.
In the client code, a `House.Builder` instance is created, and the various attributes of the house are set using the builder's fluent API. Finally, the `build()` method is called to obtain the constructed `House` object.
Running the code will produce the following output:
House details:
Foundation: Concrete
Structure: Brick
Roof: Shingles
Interior: Modern
The Builder pattern allows for the construction of complex objects by providing a step-by-step process. It allows the client code to specify only the required attributes and provides a flexible way to handle optional parameters. This pattern promotes code readability, avoids large constructor parameter lists, and enables the creation of immutable objects.
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