Question 1

What is the Observer pattern?

Accepted Answer

A pattern where objects subscribe to and are automatically notified of state changes in another object (subject/publisher). Subject maintains a list of observers. When state changes, it calls notify() on all observers. Used in event systems, MVC (Model notifies View), and reactive programming. Java has built-in Observable; most frameworks implement this pattern.

Question 2

What is the Decorator pattern?

Accepted Answer

A pattern that dynamically adds behavior to objects by wrapping them — without modifying the original class. Decorators wrap an object implementing the same interface. Coffee + MilkDecorator + SugarDecorator — each adds behavior. Java I/O streams use Decorator extensively (BufferedReader wraps FileReader).

Question 3

What is the Open/Closed Principle (OCP)?

Accepted Answer

Software entities should be open for extension but closed for modification — add new behavior via new code, not by changing existing code. Instead of modifying existing classes, extend them via inheritance or composition. Example: add a new shape without changing the draw() loop — just add a new Shape subclass. Reduces risk of breaking existing functionality.

Question 4

What is dependency injection (DI)?

Accepted Answer

Providing a class's dependencies from outside rather than having the class create them — enabling loose coupling and easier testing. Without DI: class UserService { DB db = new DB(); } — tightly coupled. With DI: class UserService { UserService(DB db) { this.db = db; } } — DB is injected, making it easy to swap in a mock DB for testing.

Question 5

What is the Strategy pattern?

Accepted Answer

A pattern that defines a family of algorithms, encapsulates each one, and makes them interchangeable at runtime without changing the client. Sort by name, price, or rating — each is a Strategy. The context holds a reference to a Strategy interface and delegates to it. Eliminates large if/else chains. Related to the Open/Closed Principle.

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