Object-Oriented Programming & Design Patterns Interview Questions and Answers (2025) | JaganInfo

OOP is a programming paradigm based on the concept of objects, which encapsulate data and behaviors, enabling modular, reusable, and organized code.

A class is a blueprint or template defining properties and methods; an object is an instance of a class with specific values.

Encapsulation is the concept of hiding internal data of an object and exposing only selected methods to manipulate the data, improving security and abstraction.

Inheritance allows a new class (child) to acquire properties and behaviors from an existing class (parent), promoting code reuse and hierarchy.

Polymorphism means many forms; it allows methods to perform differently based on the object’s class, e.g., method overloading and overriding.

Abstraction focuses on exposing only essential features and hiding complexity from users, facilitating easier interaction with objects.

A constructor is a special method invoked when an object is created to initialize its attributes.

Interfaces define a contract with abstract methods; classes implementing interfaces must provide implementations. Abstract classes can have implemented methods and state, serving as base classes.

Access modifiers control the visibility of class members; common ones include public, private, protected, and package-private.

OOP models real-world entities, promotes modularity, reusability, maintainability, and makes complex software easier to manage than procedural approaches.

Design principles include SOLID (Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion) and DRY (Don’t Repeat Yourself) to create maintainable, extensible software.

Favoring composition (has-a relationships) rather than inheritance (is-a) reduces tight coupling and increases flexibility in design.

Overloading: Same method name with different parameters in the same class (compile-time polymorphism). Overriding: Subclass redefines a superclass method with same signature (runtime polymorphism).

Encapsulation restricts direct access to object internals, preventing unintended manipulation and enforcing controlled interaction through methods.

Association: General relationship between objects. Aggregation: Whole-part relationship, parts can exist independently. Composition: Strong ownership, parts’ lifetime depends on whole.

Static methods belong to the class and can be called without an object; instance methods require an object to invoke and can access instance data.

Creation via constructor, usage with state changes, and destruction by garbage collector or explicit resource release.

Tight coupling indicates high dependency between modules (hard to maintain). Loose coupling fosters independence, ease of change, and reusability.

Keywords like public, private, protected define accessibility of classes, methods, and variables, controlling encapsulation and protection.

Multiple inheritance is a class inheriting from more than one superclass. Some languages (e.g., C++) support it directly; others (e.g., Java) use interfaces to achieve similar behavior.

When a subclass defines a static method with the same signature as a superclass, hiding the superclass’s method.

Final restricts classes from being subclassed or methods from being overridden and variables from reassignment.

Using interfaces, dependency injection, and design principles that reduce inter-module dependencies.

Abstract classes provide a partial implementation and force subclasses to implement abstract methods, enabling code reuse with enforced polymorphism.

Interfaces declare method signatures without implementation, enabling multiple inheritance behavior and decoupling.

An object’s suitability is determined by presence of methods/properties rather than its class/type. Used in dynamic languages like Python.

Subclass redefines a parent class’s method with same signature for runtime polymorphism.

The process of selecting which implementation of a polymorphic operation to call at runtime.

The method’s name, parameter types, and order which uniquely identify it in a class.

One constructor calls another constructor in the same class/type hierarchy to reuse initialization code.

SOLID stands for:

• S: Single Responsibility Principle – One class, one reason to change.
• O: Open/Closed Principle – Software entities should be open for extension but closed for modification.
• L: Liskov Substitution Principle – Subtypes must be substitutable for their base types.
• I: Interface Segregation Principle – Many client-specific interfaces are better than one general interface.
• D: Dependency Inversion Principle – Depend on abstractions, not concrete implementations.

Dependency Injection is a design pattern to pass dependencies to objects rather than creating them internally. Types include constructor injection, setter injection, and interface injection.

They provide proven, reusable solutions to common design problems, improving code readability, maintainability, and scalability.

Creational patterns deal with object creation mechanisms:

• Singleton: Ensures a class has only one instance.
• Factory Method: Creates objects without specifying exact class.
• Builder: Constructs complex objects step by step.

These patterns ease design by identifying simple ways to realize relationships between entities:

• Adapter: Allows incompatible interfaces to work together.
• Decorator: Adds responsibilities at runtime without modifying original object.
• Proxy: Controls access to another object.

Behavioral patterns focus on communication between objects:

• Observer: Objects subscribe to and receive updates from another object.
• Strategy: Encapsulates interchangeable algorithms.
• Command: Encapsulates requests as objects.

The GoF refers to Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides who authored the seminal “Design Patterns” book categorizing 23 classic software design patterns.

Objects should communicate narrowly, only with immediate friends, to reduce coupling and increase maintainability.

IoC is a design principle where the control of object creation and binding is transferred to a container or framework instead of being handled directly by the program.

Design patterns are general reusable solutions or blueprints; frameworks are concrete implementations that enforce a particular architecture or pattern.

Dividing a program into distinct features so that each section addresses a separate concern, enhancing modularity and maintainability.

By ensuring a class or module has only one reason to change, it makes code easier to understand, test, and refactor.

DIP states that high-level modules should not depend on low-level modules; both should depend on abstractions, reducing coupling and enhancing flexibility.

Coupling is the degree of interdependence between modules (lower is better). Cohesion measures how strongly-related the tasks within a single module are (higher is better).

Decorator adds new behavior to objects dynamically without altering their structure. Useful for extending functionalities like adding scrollbars to windows.

Observers subscribe to subjects to get notified automatically of any state changes, facilitating event-driven communication.