Can you name the main ways to implement popular…

Main ways to implement popular design patterns in Java:

Singleton: a class with a private constructor and a static method to get the single instance.

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Factory Method: defines an interface for creating an object, allowing subclasses to alter the type of objects that will be created.

public interface Product {}

public class ConcreteProduct implements Product {}

public abstract class Creator {
    public abstract Product factoryMethod();
}

public class ConcreteCreator extends Creator {
    public Product factoryMethod() {
        return new ConcreteProduct();
    }
}

Observer: an object (observer) subscribes to events of another object (subject) and receives notifications about changes.

public interface Observer {
    void update();
}

public class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void attach(Observer o) { observers.add(o); }
    public void notifyObservers() {
        for (Observer o : observers) {
            o.update();
        }
    }
}

Decorator: dynamically adds new responsibilities to objects by wrapping them.

public interface Component {
    void operation();
}

public class ConcreteComponent implements Component {
    public void operation() { System.out.println("Main operation"); }
}

public class Decorator implements Component {
    protected Component component;
    public Decorator(Component c) { this.component = c; }
    public void operation() {
        component.operation();
    }
}

public class ConcreteDecorator extends Decorator {
    public ConcreteDecorator(Component c) { super(c); }
    public void operation() {
        super.operation();
        System.out.println("Additional behavior");
    }
}

These are just a few examples, but in Java, most patterns are implemented through interfaces, abstract classes, and composition.

Can you name the main ways to implement popular design patterns?