What design patterns have you used in your practice?

In Java practice, the following design patterns are often used:

• Singleton — ensures that a class has only one instance and provides a global point of access to it.
• Factory Method — defines an interface for creating an object, allowing subclasses to decide which class to instantiate.
• Observer — allows objects to subscribe to events of another object and receive notifications of changes.
• Decorator — dynamically adds responsibilities to objects by wrapping them.
• Strategy — defines a family of algorithms, encapsulates each one, and makes them interchangeable.

Example of Singleton:

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

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

These patterns help write flexible, extensible, and maintainable code.