Object-Oriented Programming Interview Questions

Object-Oriented Programming (OOP) is a programming paradigm that organizes software design around objects and classes. It focuses on encapsulation, inheritance, polymorphism, and abstraction.

A class is a blueprint or template for creating objects. It defines the properties (fields) and behaviors (methods) that objects of the class will have.

An object is an instance of a class. It contains state represented by fields and behavior represented by methods.

Encapsulation is the process of binding data and methods together into a single unit and restricting direct access to the internal state of an object using access modifiers.

Encapsulation is achieved by declaring class variables as private and providing public getter and setter methods to access and modify them.

Inheritance is a mechanism that allows one class to acquire the properties and behaviors of another class, promoting code reuse and hierarchy.

Java supports single inheritance, multilevel inheritance, and hierarchical inheritance through classes. Multiple inheritance is supported through interfaces.

Java avoids multiple inheritance with classes to prevent ambiguity issues such as the Diamond Problem, where a class could inherit conflicting implementations from multiple parent classes.

Polymorphism allows objects to take multiple forms. It enables a single interface to represent different underlying implementations.

Method overloading is compile-time polymorphism where multiple methods have the same name but different parameter lists within the same class.

Method overriding is runtime polymorphism where a subclass provides a specific implementation of a method already defined in its parent class.

Overloading occurs within the same class with different parameter lists and is resolved at compile time. Overriding occurs in inheritance and is resolved at runtime.

Abstraction is the process of hiding implementation details and exposing only the essential features of an object.

Abstraction focuses on hiding implementation complexity, while encapsulation focuses on hiding internal object state and protecting data.

Follow release notes, RFCs or PEPs, official blogs, and reputable courses. Experiment in side projects when new Object-Oriented Programming-related features ship so you can speak confidently about migration paths.

Name concrete APIs (collections, concurrency, streams, annotations) tied to Object-Oriented Programming. Interviewers want syntax-level clarity plus when you would reach for each feature in production.

Prefer specific checked/unchecked exceptions, wrap lower-level failures with context, and avoid empty catch blocks. For Object-Oriented Programming, decide what is recoverable vs fatal to the request thread.

JUnit 5 with Arrange-Act-Assert, Mockito for collaborators, and parameterized tests for edge cases. Keep Object-Oriented Programming logic pure where possible so tests stay fast.

Incorrect equals/hashCode break HashMap/HashSet behavior. If Object-Oriented Programming stores objects in collections, explain the contract and mutability pitfalls.

Retain only needed references, prefer streaming for large datasets, and avoid accidental caches. Relate leaks or GC pressure to a concrete Object-Oriented Programming scenario.