Java Design Pattern Interview Questions and Answers for 5 years experience

1. What are design patterns and why are they important?

   • Answer: Design patterns are reusable solutions to commonly occurring problems in software design. They provide a common vocabulary for developers to discuss and understand solutions, promoting code reusability, maintainability, and readability. They help avoid reinventing the wheel and lead to more robust and scalable applications.

2. Explain the difference between Creational, Structural, and Behavioral design patterns. Give examples of each.

   • Answer: Creational patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. Examples include Singleton, Factory, Abstract Factory, Builder, Prototype. Structural patterns concern class and object composition. Examples include Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy. Behavioral patterns characterize the ways interacting objects distribute responsibility. Examples include Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor.

3. Describe the Singleton pattern and its uses. What are its drawbacks?

   • Answer: The Singleton pattern restricts the instantiation of a class to one "single" instance. It's useful for managing resources (like database connections), logging, or configurations where only one instance should exist. Drawbacks include testing difficulties (due to static nature) and potential for tight coupling. In modern development, dependency injection often provides better alternatives.

4. Explain the Factory pattern. When would you use a Factory vs. an Abstract Factory?

   • Answer: The Factory pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. An Abstract Factory provides an interface for creating families of related or dependent objects without specifying their concrete classes. Use Factory when you need to create a single object of various subtypes. Use Abstract Factory when you need to create families of related objects.

5. Illustrate the Builder pattern with a code example (e.g., building a car).

   • Answer: (Provide a concise Java code example demonstrating the Builder pattern for building a car object, showing how it separates the construction from the representation.)

6. Describe the Adapter pattern and provide a real-world analogy.

   • Answer: The Adapter pattern converts the interface of a class into another interface clients expect. It lets classes work together that couldn't otherwise because of incompatible interfaces. A real-world analogy is a power adapter for electronic devices; it adapts the power source to the device's needs.

7. Explain the Decorator pattern and its advantages over inheritance.

   • Answer: The Decorator pattern dynamically adds responsibilities to an object. It provides a flexible alternative to subclassing for extending functionality. Advantages over inheritance include adding responsibilities without modifying the original class and allowing for multiple, independent additions of functionality.

8. What is the Observer pattern? Give a practical example.

   • Answer: The Observer pattern defines a one-to-many dependency between objects where a state change in one object (subject) automatically notifies its dependents (observers). A practical example is a stock ticker application where multiple clients observe changes in a stock's price.

9. Describe the Strategy pattern and how it promotes loose coupling.

   • Answer: The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. This promotes loose coupling because the client doesn't need to know the specifics of the algorithm being used; it only interacts with a common interface.

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