Tell about design patterns — why they are needed, what groups, examples.

Design patterns are proven solutions to common problems in software development. They help create flexible, scalable, and maintainable architectures.

Main groups of patterns:

• Creational — responsible for object creation, hiding instantiation details. Examples: Singleton, Factory Method, Abstract Factory, Builder, Prototype.
• Structural — define how classes and objects are combined into larger structures. Examples: Adapter, Decorator, Composite, Proxy, Facade.
• Behavioral — describe interactions and responsibilities distribution among objects. Examples: Observer, Strategy, Command, Iterator, State.

Example of Singleton pattern in Java:

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

Patterns help standardize architecture, simplify communication among developers, and accelerate development by reusing proven solutions.