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OOP JAVA interview
| Term | Definition |
|---|---|
| What is Object-Oriented Programming | A way of designing real world entities as objects that bundle data and behavior, making code modular, reusable, and maintainable |
| What is a class in Java | A user defined blueprint that defines attributes and methods used to create objects |
| What is an object in Java | An instance of a class that holds real data and represents a specific entity |
| What are the four principles of OOP | Encapsulation Inheritance Polymorphism and Abstraction |
| What is encapsulation | Bundling data and methods together and restricting direct access using private fields and public methods |
| Why is encapsulation important | It protects sensitive data and ensures controlled access through methods |
| What is abstraction | Showing only essential functionality while hiding implementation details using abstract classes or interfaces |
| Why is abstraction useful | It reduces complexity and lets users focus on what an object does instead of how it works |
| What is inheritance | allows a class to reuse and extend the properties and behavior of another class |
| When should inheritance be used | When there is a true is-a relationship between classes like a manager is an employee |
| What is polymorphism | it means same method name, different behavior through overloading and overriding |
| What are the types of polymorphism | Method overloading at compile time and method overriding at runtime |
| What is method overloading | Multiple methods with the same name but different parameter lists in the same class |
| What is method overriding | A subclass providing a new implementation of a parent class method |
| Difference between overloading and overriding | Overloading is compile time in same class overriding is runtime in parent child classes |
| What is an abstract class | A base class with shared state and behavior that can have both abstract and implemented methods |
| What is an interface | a contract that defines methods a class must implement and allows multiple inheritance. |
| Difference between abstract class and interface | Abstract class can have state and partial implementation interface defines only behavior and supports multiple implementation |
| Can a class implement multiple interfaces | Yes a class can implement multiple interfaces |
| Can a class extend multiple classes | No Java does not support multiple inheritance with classes |
| What is a constructor | A special method that initializes an object when it is created |
| Can constructors be overloaded | Yes constructors can have different parameter lists |
| What is this keyword | Refers to the current object and is used to access instance members or resolve conflicts |
| What is super keyword | Refers to the parent class and is used to call parent methods or constructors |
| What is static keyword | Belongs to the class rather than instances and is shared across all objects |
| What is final keyword | Prevents modification of variables methods or classes |
| What are access modifiers in Java | Keywords that control visibility of classes methods and variables (ex: private, default, protected, and public) |
| What is private access modifier | Accessible only within the same class |
| What is default access modifier | Accessible only within the same package |
| What is protected access modifier | Accessible within the same package and in subclasses outside the package |
| What is public access modifier | Accessible from anywhere |
| What is association in OOP | A general relationship where classes are connected but independent |
| What is aggregation in OOP | A weak has a relationship where the child can exist independently of the parent |
| What is composition in OOP | A strong has a relationship where the child depends on the parent for existence |
| Difference between aggregation and composition | Aggregation allows independent existence while composition does not |
| What is dynamic method dispatch | Runtime mechanism where the method to execute is decided based on the actual object |
| Why is polymorphism powerful | It allows flexible and extensible code by enabling one interface to support multiple behaviors |
| What is composition over inheritance | A design principle that prefers combining objects over inheriting to reduce coupling and increase flexibility |
| Why prefer composition over inheritance | It provides more flexibility and allows behavior to change dynamically |
| What does == do in Java | Checks if two references point to the same object |
| What does equals() method do | Checks logical equality of object contents |
| What is hashing in Java | Converting data into a fixed value for fast lookup |
| What is HashMap | A collection that stores key value pairs using hashing for fast retrieval |
| What is hashCode method | Generates a hash value used to store and retrieve objects efficiently |
| What is String immutability | Strings cannot be changed after creation and any modification creates a new object |
| Why are strings immutable | For security performance and thread safety |
| What is Java Collections Framework | A set of interfaces and classes to store and manage groups of objects |
| What is List interface | An ordered collection that allows duplicates |
| What is Set interface | A collection that stores unique elements |
| What is Queue interface | A collection that follows FIFO order |
| What is Map interface | A collection that stores key value pairs with unique keys |
| What is ArrayList | A dynamic array with fast access but slower insertions |
| What is LinkedList | A linked structure with fast insertions but slower access |
| What is HashSet | A collection that stores unique elements without order |
| What is TreeSet | A collection that stores unique elements in sorted order |
| What is Comparable interface | Defines natural ordering using compareTo method |
| What is Comparator interface | Defines custom sorting logic |
| What is Big O notation | A measure of algorithm performance based on input size |
| Why is Big O important | It helps evaluate efficiency and scalability of code |
| Can we create object of abstract class | No it cannot be instantiated directly and must be extended |
| What is compile time polymorphism | Achieved using method overloading |
| What is runtime polymorphism | Achieved using method overriding |
| What is static binding | Method call is resolved at compile time |
| What is dynamic binding | Method call is resolved at runtime based on object |
| When to use abstract class | When you need shared state and partial implementation |
| When to use interface | When you need a contract and multiple inheritance |
| What are types of constructors | Default constructor and parameterized constructor |
| what is Default constructor | A constructor with no parameters |
| what is Parameterized constructor | A constructor that takes arguments to initialize values |
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