Object Oriented Programming
General
Q1: What are the four pillars of OOP in Java?
A: Encapsulation, Inheritance, Polymorphism, and Abstraction.
Q2: What is the difference between a class and an object?
A: A class is a blueprint or template. An object is an instance of that class with actual data.
Q3: What is the difference between == and equals() in Java?
A: == compares object references. equals() compares object values when properly overridden.
Encapsulation
Q4: What is encapsulation?
A: Encapsulation hides internal implementation details and exposes only necessary operations through methods. It is usually achieved with private fields and public getters/setters.
Q5: Why is encapsulation important?
A: It protects object state, improves maintainability, and prevents unauthorized modification of data.
Q6: What is the effect of making a field private?
A: A private field is accessible only within its own class. External classes must use public methods to read or modify it.
Inheritance
Q7: Explain inheritance in Java.
A: Inheritance allows a subclass to inherit properties and methods from a superclass. It promotes code reuse and enables a class hierarchy.
Q8: What is the difference between extends and implements?
A: extends is used to inherit from a superclass. implements is used to adopt one or more interfaces.
Q9: Can a Java class extend more than one class?
A: No. Java does not support multiple inheritance for classes. A class can extend only one superclass but implement multiple interfaces.
Polymorphism
Q10: What is polymorphism?
A: Polymorphism means "many forms." In Java, it lets a variable refer to objects of different types and call overridden methods dynamically.
Q11: What is method overloading?
A: Method overloading is defining multiple methods with the same name but different parameter lists in the same class.
Q12: What is method overriding?
A: Method overriding occurs when a subclass provides its own implementation of a method already defined in its superclass.
Abstraction
Q13: What is abstraction?
A: Abstraction focuses on exposing only essential features while hiding implementation details. It is achieved using abstract classes and interfaces.
Q14: What is the difference between abstract class and interface?
A: Abstract classes can have state and concrete methods, and a class can extend one abstract class. Interfaces can have default/static methods and a class can implement multiple interfaces.
Q15: When should you use an abstract class instead of an interface?
A: Use an abstract class when you need shared code or state among related classes. Use interfaces to define a contract that multiple unrelated classes can implement.
Classes and Interfaces
Q16: What is a functional interface?
A: A functional interface has a single abstract method. It can be used with lambda expressions and method references.
Q17: What is the purpose of the super keyword?
A: super refers to the superclass instance. It can be used to call superclass constructors or methods.
Q18: What is the role of constructors in inheritance?
A: A subclass constructor calls its superclass constructor first, either implicitly or with super(...), to initialize inherited state.
Q19: What is a nested class?
A: A nested class is declared inside another class. It can be static (nested static class) or non-static (inner class), and it helps organize related code.
Common interview focus
Q20: What is the difference between compile-time and runtime polymorphism?
A: Compile-time polymorphism is method overloading, resolved at compile time. Runtime polymorphism is method overriding, resolved during program execution.
Q21: What is the Liskov Substitution Principle?
A: It states that objects of a superclass should be replaceable with objects of a subclass without affecting program correctness.